Key Takeaways
- Email blocklisting can happen even if you don’t intentionally send spam — poor practices alone can trigger it.
- Purchased lists are one of the fastest ways to get blocklisted due to spamtraps and disengaged recipients.
- Double opt-in helps guarantee real, valid signups and minimizes spam complaints.
- Repetitive, vague, or misleading content increases complaint rates and damages sender reputation.
- Attachments are commonly blocked at gateways and should be avoided in bulk or marketing campaigns.
- Proper unsubscribe methods, sender identity, and subscriber trust signals significantly affect deliverability.
- Sending frequency must be consistent but not excessive to maintain engagement and avoid filtering.
- Strong list hygiene practices prevent bounces, protect reputation, and maintain healthy deliverability.
- Avoid spammy signals such as excessive punctuation, all-caps, image-only messages, and BCC sending.
- Securing your sending environment is essential — malware-infected servers can send spam without your knowledge.
Getting your email blocked or landing on an email blocklist has a direct impact on your email deliverability. Unfortunately, you don’t have to send spam to have your email blocked or be blocklisted.
A blocklist is created by independent spam organizations who put together a list of emails and IPs that have been previously seen sending spam. In fact, some ISPs (and other companies) even have their own internal blocklists. As a result, every email administrator has the ability to choose to block things that are on the blocklist and they can also block any other emails or IPs they think look spammy.
Here are 15 do’s and don’ts to avoid having your email blocked or blocklisted:
- Don’t buy email lists. Ever. The people on these lists are likely to mark your unsolicited emails as spam, and there’s a good chance that a spamtrap is included in the list. Sending email to a spamtrap will usually land you on a blocklist.
- Do use double opt-in to confirm email list subscribers. This helps ensure that subscribers are signing up with their own email addresses and that those email addresses are valid.
- Don’t repeatedly send the same or similar content. Every email you send should consist of unique content. Not only will this help keep you stay off of blocklists, but it will also keep your subscribers engaged.
- Don’t string readers along with vague content. Include a clearly written subject line and call to action.
- Don’t send attachments, it’s a sure way to get blocked. (Sending attachments may get your email blocked, but not blocklisted. The effect on deliverability may be the same, but the remedy is to simply not send attachments rather than working to get removed from a list).
- Do include information on how to unsubscribe in every email.
- Do watch your sending frequency. Send email regularly while being careful not to overwhelm readers.
(we recommend at a bare minimum to send a message at least once every six months to keep your email list fresh. Unless you are a daily sender like Groupon, most email senders should avoid sending more than once or twice a week.) - Do send email from a legitimate address that is checked by a real person. Email addresses with random letters and numbers trigger spam filters and can lead to your email being blocked.
- Do ask subscribers to add your email address to their contact list.
- Do practice good list hygiene. Regularly clean your email subscriber list to avoid repeatedly sending email to bad or non-existent email addresses.
- Don’t use punctuation (such as exclamation marks) or words that are often used by spammers. These include free, win, and opportunity.
- Don’t blind carbon copy (BCC) your list.
- Don’t use too many images. Strive for a healthy balance of images and text to avoid triggering spam filters.
- Don’t use all caps in your subject line – or any other part of your email. Instead, use bold, italics and underline to show emphasis.
- Do protect your email server from malware. An infected email server can be used as part of a botnet to send spam.
(We once had a prospect come to us with severe deliverability problems not explained by their mailing practices. When we looked into it, we found that their web server had been infected and was sending out email spam. They’d had no idea. Once we cleaned that up, their deliverability improved a great deal.)
Summary of Deliverability Risks
| Rule Category | Do / Don’t | Deliverability Impact |
|---|---|---|
| List acquisition | Don’t buy lists | High risk of spamtraps, complaints, instant blocklisting |
| Consent & signup quality | Do use double opt-in | Ensures valid signups and reduces spam complaints |
| Content freshness | Don’t repeat the same content | Prevents low-engagement patterns that damage reputation |
| Clarity & honesty | Don’t use vague content | Reduces complaints and spam filtering due to misleading messages |
| Attachments | Don’t send attachments | Attachments trigger gateway blocks even when not malicious |
| Compliance | Do include unsubscribe links | Required for compliance and prevents complaints |
| Sending frequency | Do send at a healthy cadence | Avoids list decay and sudden engagement drops |
| Sender identity | Do send from a legitimate address | Looks trustworthy and avoids bot-like patterns |
| Whitelisting | Do ask users to add you to contacts | Improves inbox placement |
| List hygiene | Do maintain a clean list | Reduces bounces and prevents blocklist escalations |
| Spammy wording | Don’t use spam-trigger words | Bypasses linguistic filters and improves inboxing |
| Sending method | Don’t BCC the list | Looks like spam blasts and triggers filtering |
| Email composition | Don’t use too many images | Avoids image-heavy spam patterns |
| Formatting style | Don’t use all caps | Reduces spam scoring based on aggressive formatting |
| Security | Do protect sending servers | Prevents malware/botnet activity that auto-causes blocklisting |
Q&A
What causes an email to be blocked or blocklisted?
ISPs and anti-spam organizations maintain blocklists of IPs and domains associated with spam-like behavior. Poor sending practices, spamtraps, high complaint rates, and infected servers can all result in blocks—even for legitimate senders.
Why is buying email lists so dangerous?
Purchased lists often contain spamtraps, stale addresses, and people who never opted in. Sending to them leads to high complaint rates and almost guarantees blocklisting.
How does double opt-in help prevent blocklisting?
Double opt-in ensures subscribers confirm ownership of their email address. This reduces fake signups, prevents complaints, and improves list quality.
Why is repetitive or vague content risky?
Sending the same or similar emails triggers engagement drop-offs. Low engagement signals can cause mailbox providers to classify your emails as unwanted or spam.
Why should senders avoid attachments?
Attachments are often blocked at gateways for security reasons. This results in failed deliveries—and while not always a blocklist issue, the impact on deliverability is similar.
Why is including an unsubscribe link essential?
Hidden or missing opt-outs frustrate users, leading to spam complaints. These complaints directly damage sender reputation and trigger filtering.
How does sending frequency affect deliverability?
Infrequent sends lead to list decay, while over-sending overwhelms subscribers. A steady cadence keeps engagement healthy and reduces the likelihood of complaints.
Why does sender identity matter?
Emails sent from random or unmonitored addresses look suspicious to filters and users. Sending from a legitimate, monitored address builds trust and improves inbox placement.
What role does list hygiene play in avoiding blocks?
Removing invalid, bouncing, or disengaged recipients prevents unnecessary spam signals. Clean lists maintain strong sender reputation and reduce blocklist risk.
How can malware lead to blocklisting?
A compromised server can unknowingly send spam or phishing messages. Once detected, mailbox providers may block the server’s IP — harming all legitimate mail from that environment.