8 Email Deliverability Mistakes Costing You Revenue (And How to Fix Them)
Feb 26, 2026
Why deliverability failures happen to good campaigns
You've crafted the perfect email campaign - tested subject lines, personalized content, compelling offers. But if you're not following email deliverability best practices, thousands of those emails never reach inboxes.
The problem isn't your campaign. It's deliverability. They landed in spam folders, got blocked by ISPs, or were rejected entirely. Your subscribers never saw them.
Most marketing teams discover deliverability problems only after campaigns fail. By then, sender reputation is damaged, ISPs have flagged your domain, and fixing the problem takes weeks of remediation.
The frustrating part is that most deliverability failures are preventable. They're caused by technical oversights and strategic mistakes that accumulate over time until inbox placement collapses.
This guide covers the eight most common deliverability mistakes we see across the 40% of global commercial email that flows through Bird's infrastructure - serving 35,000+ customers with 99.99% uptime - and the specific fixes that restore inbox placement.
Mistake 1: Ignoring hard bounces and keeping dead addresses on your list
What's happening: Hard bounces occur when you send to email addresses that don't exist or domains that no longer accept mail. These addresses will never successfully receive email. Yet many marketing teams keep them on their lists, sending campaign after campaign to addresses that reject every message.
Why it kills deliverability: ISPs track your bounce rates. High bounce rates signal that you're not maintaining your list properly - a characteristic of spammers who scrape email addresses or buy lists. When ISPs see bounce rates creeping above 2-3%, they start filtering your mail more aggressively. Above 5%, you're at serious risk of being blocked entirely.
Every hard bounce damages your sender reputation. Keep sending to those addresses and you're telling ISPs you either don't know what you're doing or don't care about sending to real people.
The fix:
Remove hard bounces immediately. Don't wait for monthly list cleaning. Addresses that hard bounce should be suppressed automatically before your next send.
Set up automated suppression rules that prevent sending to any address that's hard bounced more than once.
Monitor your bounce rate by campaign. If a campaign has a bounce rate above 2%, investigate why before sending again.
Check for typos in bulk imports. Common mistakes like "gmial.com" instead of "gmail.com" create hard bounces that could have been caught with basic validation.
Run your list through a tool like Recipient Validation to verify email addresses are valid, active, and able to receive messages before they are sent
Most email platforms provide bounce categorization (hard vs. soft). Use it. Hard bounces should be immediately suppressed. Soft bounces (mailbox full, temporary issues) can be retried, but after three consecutive soft bounces, setup suppression rules to prevent further sends.
Mistake 2: Inconsistent sending volume that triggers ISP filters
What's happening: You send 10,000 emails per week for months, then suddenly send 500,000 for a product launch. Or you go quiet for three weeks, then resume sending at your normal volume. These dramatic volume changes trigger ISP spam filters.
Why it kills deliverability: ISPs use sending patterns to identify legitimate senders. Established senders have predictable patterns. Spammers have erratic patterns, quiet periods followed by massive blasts.
When your volume suddenly spikes, ISPs don't see a product launch. They see suspicious behavior that matches spam patterns. Even if you're sending to engaged subscribers, sudden volume changes cause filtering.
The fix:
Ramp volume gradually on already-warmed IPs. If you normally send 50,000 emails weekly and need to send 500,000 for a product launch, split it over 3-4 days rather than sending all at once — 100,000 the first day, 150,000 the second, 250,000 the third. This signals to ISPs it's a planned campaign, not erratic spammer behavior. (Note: this is different from IP warming, which takes 6+ weeks when first setting up a dedicated IP.)
Maintain consistent sending schedules. If you send campaigns every Tuesday and Thursday, keep that pattern even during slow periods. Send smaller campaigns rather than going completely silent.
Plan for seasonal spikes in advance. If Black Friday means 10X normal volume, start ramping volume 2-3 weeks before, not the day of.
Use separate sending infrastructure for transactional versus marketing emails - both dedicated IPs AND separate subdomains. Send marketing from marketing.yourdomain.com and transactional emails (order confirmations, password resets) from transactional.yourdomain.com. This isolates reputations completely. If marketing campaigns trigger spam complaints, your critical transactional emails remain unaffected.
ISPs give you more leeway for volume increases when you've established consistent patterns over time. A sender who's been reliable for six months can spike more safely than a sender with erratic history.
Mistake 3: Missing or misconfigured email authentication
What's happening: Email authentication (SPF, DKIM, DMARC) proves to ISPs that you're authorized to send from your domain. Many marketing teams either don't implement these protocols or configure them incorrectly, leaving their emails unauthenticated.
Why it kills deliverability: Unauthenticated email is the digital equivalent of an unsigned letter. ISPs can't verify you're actually who you claim to be. Gmail and Yahoo made authentication mandatory for bulk senders sending 5,000+ messages per day in February 2024. Outlook is moving in the same direction.
Even partial authentication isn't enough. SPF without DKIM still leaves verification gaps. DMARC without proper SPF and DKIM monitoring can cause legitimate mail to be rejected.
The fix:
Implement SPF (Sender Policy Framework) to specify which mail servers can send from your domain. Add all legitimate sending IPs to your SPF record.
Configure DKIM (DomainKeys Identified Mail) to cryptographically sign your emails. This proves the message wasn't altered in transit and came from your domain.
Set up DMARC (Domain-based Message Authentication, Reporting and Conformance) to tell ISPs what to do with emails that fail authentication checks. Start with a monitoring policy (p=none) to see what's failing before moving to reject.
Monitor DMARC reports to catch authentication failures. These reports show you when emails are failing SPF or DKIM checks, letting you fix configuration issues before they impact deliverability.
Verify your authentication is working using tools like mail-tester.com before launching campaigns.
Authentication isn't optional anymore. Major ISPs now require it for bulk senders. If you're sending marketing email without proper SPF, DKIM, and DMARC, you're fighting deliverability with a fundamental disadvantage.
Mistake 4: Relying on shared IP pools without reputation control
What's happening: Most email platforms use shared IP pools where your emails are sent from IP addresses used by hundreds or thousands of other customers. Your sender reputation gets mixed with everyone else's behavior on those IPs.
Why it kills deliverability: If another sender on your shared IP gets flagged for spam or hits spam traps, your deliverability suffers too. You're not responsible for their behavior, but you share the reputational consequences because you share the infrastructure.
Shared IPs work fine for low-volume senders, but once you're sending hundreds of thousands of emails monthly, you need control over your sender reputation.
The fix:
Request dedicated IP addresses once you're sending 50k emails or more per month consistently. Dedicated IPs give you complete control over sender reputation, but new IPs start with zero reputation and must be warmed gradually.
Warm up dedicated IPs gradually. Don't immediately send full volume from a new IP. Start with your most engaged subscribers and slowly increase volume over 6-8 weeks.
Monitor your IP reputation using tools like Sender Score or Google Postmaster Tools. Check reputation weekly to catch problems before they escalate.
If using shared IPs (which work well for senders under 50k monthly volume), choose platforms that segment pools by sender quality. Some platforms separate high-reputation senders from problematic ones, limiting the impact of bad actors.
The tradeoff with dedicated IPs is you're fully responsible for maintaining reputation. There's no hiding behind a shared pool. But for serious senders, that control is worth the responsibility.
Mistake 5: Sending to unengaged subscribers who never open your emails
What's happening: Your list includes thousands of subscribers who haven't opened an email in six months, a year, or ever. You keep sending to them anyway, hoping they'll eventually engage.
Why it kills deliverability: ISPs track engagement signals - opens, clicks, replies, moving emails to folders. When you consistently send to addresses that never engage, ISPs interpret that as unwanted mail. Low engagement rates signal that recipients don't value your emails, which leads to filtering.
Engagement matters more than list size. A 100,000-subscriber list with 40% engagement delivers better results than a 500,000-subscriber list with 8% engagement.
The fix:
Segment subscribers by engagement level. Identify who's opened or clicked in the last 30, 60, 90 days. Send different frequencies to each segment.
Reduce send frequency to unengaged subscribers before removing them entirely. Instead of weekly emails, try monthly. If they don't engage with reduced frequency, they won't engage with increased frequency.
Run re-engagement campaigns with compelling subject lines before suppressing inactive subscribers. Give them one final chance to confirm interest.
Suppress subscribers who haven't engaged. Some platforms recommend 90 days, others 180. The key is having a policy and enforcing it.
Remove subscribers who haven’t engaged/opened an email in the last year.
Monitor engagement rates by campaign using click-through rates, not open rates. Since Apple Mail Privacy Protection reports false opens, clicks are the reliable engagement signal. If click rates drop more than 20% below your baseline, investigate what's causing disengagement before sending again. Track benchmarks by industry- B2B SaaS averages 2-3% CTR, retail averages 1-2%.
It feels counterintuitive to remove subscribers, but cleaning your list improves deliverability for the subscribers who actually want your emails. Better to reach 50,000 engaged inboxes than land in spam for 200,000 uninterested addresses.
Mistake 6: Using "no-reply" sender addresses that discourage engagement
What's happening: You're sending campaigns from addresses like "no-reply@yourdomain.com" or "donotreply@yourdomain.com" that can't receive responses.
Why it kills deliverability: ISPs favor emails that generate two-way conversation. Reply rates are a positive engagement signal - secondary to opens, clicks, and folder moves, but one ISPs do register. When you use no-reply addresses, you're explicitly preventing the engagement signal that helps deliverability.
Beyond deliverability, no-reply addresses create poor user experience. When customers try to reply with questions or feedback and get delivery failures, you've broken communication.
The fix:
Use a real sender address, but route replies to a separate monitored inbox. Send FROM marketing@yourdomain.com but set replies to go TO hello@yourdomain.com or support@yourdomain.com. This prevents out-of-office replies and bounce notifications from cluttering your sending address while ensuring genuine replies reach a monitored inbox.
Consider using personal sender names and addresses for different campaign types. "sarah@yourdomain.com" from your CEO for company updates feels more legitimate than generic marketing addresses.
Monitor replies and respond to genuine inquiries. At 100,000+ sends, you'll receive dozens of automated responses—out-of-office notifications, bounce messages, address-doesn't-exist errors. Set up inbox filtering to route these automated messages separately from genuine customer replies. Tools like Gmail filters or dedicated customer service platforms can categorize incoming mail automatically, letting your team focus on actual questions and feedback without drowning in noise.
Set up automatic responses for common reply types (unsubscribe requests, questions) while escalating complex issues to the right teams.
Reply rates are typically low for marketing emails - often under 0.1%. But those replies matter disproportionately for deliverability. ISPs register them as a signal that recipients want to hear from you - small in weight, but positive.
For transactional emails (order confirmations, shipping notifications, password resets), reply rates aren't a priority metric. Focus instead on open rates, click rates, and spam complaint rates. The critical factor for transactional deliverability is making it easy for customers to reply or unsubscribe when needed. If customers can't find an easy way to opt out or ask questions, they're more likely to hit 'report spam' - which damages your sender reputation far more than low reply rates ever would.
Mistake 7: Not monitoring for spam trap hits
What's happening: Spam traps are email addresses that exist solely to identify spammers. They're addresses that were once valid but have been abandoned, or addresses created specifically as traps that were never used by real people. If you're sending to spam traps, your list hygiene is problematic.
Why it kills deliverability: Spam trap hits tell ISPs you're either scraping email addresses, buying lists, or not removing inactive addresses. All are spam behaviors. Hit enough spam traps and ISPs will block your domain entirely.
The challenge is you don't know which addresses are spam traps. ISPs don't publish them. You only discover you've hit traps when deliverability collapses.
The fix:
Never buy or rent email lists. Purchased lists are guaranteed to contain spam traps.
Implement confirmed opt-in (double opt-in) for all new subscribers. This ensures the person who entered the address actually controls it.
Remove addresses that hard bounce or remain inactive for extended periods. Old, abandoned addresses often get converted into spam traps.
Work with email platforms that have relationships with ISPs and can alert you to spam trap hits before they destroy your reputation.
If you suspect spam trap hits, immediately stop sending and audit your list. Remove any addresses acquired through questionable sources.
Understanding spam trap types helps you diagnose the root cause:
Recycled spam traps are email addresses that were once valid but abandoned. ISPs repurpose them as traps after an inactive period. Hitting recycled traps signals list hygiene problems—you're not removing inactive addresses. Fix: Immediately remove subscribers who haven't engaged in 6+ months.
Pristine spam traps are addresses that were never used by real people. They appear on websites, forums, or purchased lists to catch scrapers. Hitting pristine traps signals acquisition problems - questionable sources are feeding bad addresses into your CRM. Fix: Audit every source that adds emails to your list. Stop any questionable acquisition channels immediately.
Different problems require different remediation strategies. Recycled traps mean clean your list. Pristine traps mean audit your acquisition process.
Mistake 8: Not monitoring sender reputation and waiting until delivery fails
What's happening: Most marketing teams only check deliverability when campaigns perform poorly. By then, sender reputation has already degraded and recovery takes weeks.
Why it kills deliverability: Sender reputation degrades gradually. Bounce rates creep up. Spam complaints increase slowly. Engagement drops over time. None of these changes are dramatic enough to notice day-to-day, but accumulated over weeks they destroy deliverability.
Waiting until a campaign fails means you're in reactive mode, trying to diagnose problems with incomplete data and damaged reputation.
The fix:
Monitor sender reputation weekly using tools like:
Google Postmaster Tools (set up at postmaster.google.com to track Gmail delivery data)
Microsoft SNDS (for Outlook.com delivery data)
Sender Score (for overall IP reputation)
Track deliverability metrics by campaign: inbox placement rate, spam folder rate, bounce rate, spam complaint rate.
Set up alerts for deliverability thresholds. If bounce rate exceeds 2% or complaint rate exceeds 0.1%, investigate immediately.
Review authentication reports (DMARC) regularly to catch configuration issues before they impact delivery.
Test deliverability proactively by sending to seed lists (test addresses in Gmail, Outlook, Yahoo) and checking inbox placement.
Deliverability monitoring should be as routine as checking campaign performance metrics. Reputation damage is easier to prevent than repair.
What makes these fixes stick (or fail): the infrastructure layer
The eight mistakes above are fixable regardless of what platform you're on. But how quickly you can fix them, how much visibility you have into what's breaking, and how much control you have over the outcome depends heavily on the infrastructure underneath your email program.
"There's a meaningful difference between email platforms that own their sending infrastructure and those that rent it from third-party providers like SendGrid or Amazon SES. That difference matters most exactly when deliverability problems occur."
Most email platforms rent their infrastructure. They send through SendGrid or Amazon SES or other third-party providers. This means you're sharing IP pools with thousands of other senders, relying on generic delivery infrastructure, and depending on vendors who prioritize throughput over inbox placement.
When deliverability problems occur on rented infrastructure, diagnosis is slow. You file tickets. Your platform files tickets with their provider. Days pass while you wait for answers about why emails failed. Meanwhile, your sender reputation continues degrading.
Bird owns its infrastructure - built in-house because we kept seeing the same pattern: companies implementing every best practice correctly, then hitting ceilings they couldn't diagnose or control because the infrastructure underneath them wasn't built for it.
The practical difference shows up in four areas:
Reputation isolation. On shared infrastructure, your deliverability is partially determined by what other senders on your IP pool do. On owned infrastructure, high-reputation senders are segmented from problematic ones. Your sender reputation reflects your behavior, not someone else's.
Authentication visibility. SPF, DKIM, and DMARC aren't configurations you set once and hope hold. On infrastructure built for deliverability, authentication is monitored continuously - misconfigurations surface before they affect inbox placement, not after a campaign underperforms.
ISP escalation. When Gmail or Outlook begins filtering your mail, resolution speed depends on who's making the call. Platforms with direct ISP relationships can escalate and resolve delivery issues in hours. Platforms routing through third-party providers add layers to that conversation, and days to that timeline.
Diagnosis speed. When something breaks, visibility into why requires access to the full sending stack. Owned infrastructure means complete data at every layer - no tickets filed across vendors, no waiting on third parties to surface what failed.
This is why companies that migrate to Bird consistently see 99.3% inbox deliverability. Across 35,000+ customers - representing 40% of global commercial email - the fixes in this guide work faster and hold more reliably when the infrastructure underneath them was built specifically for inbox placement, not just throughput.
Email Deliverability Best Practices: Monthly Maintenance Checklist
Good deliverability is ongoing work, not a one-time fix. Use this checklist monthly to maintain inbox placement:
☐ Remove all hard bounces from your list
☐ Review and segment by engagement (0-30 days, 30-90 days, 90+ days inactive)
☐ Check authentication (SPF, DKIM, DMARC) is properly configured
☐ Monitor sender reputation scores
☐ Review spam complaint rates (should be under 0.1%)
☐ Check bounce rates by campaign (should be under 2%)
☐ Test inbox placement with seed lists
☐ Review engagement trends to catch drops early
Deliverability is the foundation of email marketing. All the creative, all the personalization, all the automation means nothing if your emails don't reach inboxes.
Fix these eight mistakes on infrastructure built for deliverability, and you'll see the inbox placement that turns email campaigns into revenue drivers.
—
Learn more about email infrastructure:
Explore Bird's email API.
