A deferred email is a message the receiving server has temporarily declined to accept, asking the sender to try again later. It is signaled by a 4xx SMTP response, and it is not a failure: the address is usually valid and the mail is still in flight. This is the key distinction from a bounce, which is a permanent 5xx rejection. With a deferral, your sending system keeps the message and retries on a schedule until the server accepts it or gives up.
How is a deferral different from a bounce?
It comes down to the SMTP response code and what it promises. A 4xx code means "not now, try later." A 5xx code means "no, and do not bother trying again." A deferral leaves the door open; a bounce closes it.
| Deferred | Bounced | |
|---|---|---|
| SMTP response | 4xx (temporary) | 5xx (permanent) |
| Meaning | Retry later | Do not retry |
| Message state | Still queued by sender | Returned to sender |
| Typical outcome | Delivered on a later attempt | Suppressed |
Because the message is still queued rather than returned, a deferral is invisible to most recipients. The mail simply arrives a little later. For the difference on the permanent side, see what are bounced emails.
What causes an email to be deferred?
Deferrals are the receiving server's way of slowing you down or buying itself time. The common causes:
- Greylisting. The server intentionally defers the first attempt from an unfamiliar sender, on the logic that real mail systems retry and many spam tools do not. Your retry usually goes through.
- Rate limiting and throttling. Mailbox providers cap how much mail they will accept from a given sender in a window. Exceed the cap and the rest is deferred until the window resets.
- Temporary recipient-server issues. The receiving server is briefly overloaded, in maintenance, or having a transient problem. It defers rather than loses your mail.
- Reputation-based throttling. Providers throttle senders they do not yet trust, which hits new or cold domains hardest. A domain still building reputation will see more deferrals until it has a track record.
That last one is why deferrals are common during the early days of a sending domain. The fix is patience and a gradual ramp; how to warm up a new email domain covers the schedule that keeps throttling from turning into a bigger problem.
What happens after a deferral?
Your message transfer agent (the MTA, the system that actually delivers your mail) holds the deferred message and retries it on a schedule. Early retries come quickly; later ones space out, often over hours, as the MTA gives the receiving server time to clear whatever caused the delay. Most deferrals resolve on their own within the first few attempts, which is why you generally should not panic at a deferral or resend the message yourself: a manual resend just adds a duplicate to the queue.
If the receiving server keeps deferring past the retry window, the MTA eventually stops trying and converts the message into a bounce. At that point a temporary problem has become a permanent one, and the address should be treated like any other hard bounce. So a deferral is best read as a warning: a few are routine, but a flood of them, especially the reputation and throttling kind, is telling you the receiving providers are not comfortable with your volume or your standing yet.
What should a sender do about deferrals?
Mostly, let your sending platform's retry logic do its job. Beyond that, the useful moves are about reducing the throttling that causes deferrals in the first place.
- Watch the pattern, not the single event. One deferral is noise. A spike concentrated at one mailbox provider points to throttling or a reputation problem with that provider specifically.
- Slow your ramp. If a new domain or IP is getting throttled, you are sending too much too soon. Ease into volume so providers can build trust.
- Keep your sending healthy. Strong authentication, a clean list, and engaged recipients all lower the odds of reputation-based throttling. These are the same fundamentals in email deliverability best practices.
- Read your logs. Bird records each deferral with its response code and reason, so you can see exactly which provider is deferring and why.
The events guide documents the deferral and bounce event types, and the email log lets you trace an individual message through its delivery attempts.
FAQ
Is a deferred email lost?
No. A deferred message is still queued and will be retried automatically. Most deferrals clear within the first few attempts and the mail arrives a little later than usual.
Do I need to resend a deferred email?
No. Your sending system retries on its own schedule. Resending manually just creates a duplicate in the queue and can make throttling worse.
When does a deferral become a bounce?
When retries keep failing past the MTA's retry window. At that point the system gives up and converts the deferred message into a bounce, which you should then treat as a permanent failure.
Why does a new domain get so many deferrals?
Mailbox providers throttle senders they do not yet trust, and a new domain has no track record. Warming the domain up gradually builds that trust and brings the deferral rate down.
Where to go next
Deferrals are a normal part of how email moves, and the right response is usually to wait and watch. Let retries run, ramp new domains slowly, and treat a cluster of deferrals as a signal about your reputation with a specific provider. The events guide and the email log are where you will see deferrals as they happen and follow what your retries are doing.