Not every delivery failure is permanent. When a message can't be delivered right away, SMTP has a built-in mechanism to try again later rather than giving up immediately. If you've ever seen a message sitting in a mail queue for a while before eventually going through — or bouncing after several attempts — you've run into SMTP retries and deferrals.
Retries vs. deferrals
- Retries: When a message fails on its first attempt due to a temporary problem, the sending server doesn't treat it as a permanent failure. Instead, it automatically tries again after a set interval.
- Deferrals: When the receiving server isn't ready to accept a message, it returns a temporary error code (typically in the 4xx range) and the message is placed in a queue to be retried later rather than rejected outright.
How the retry process works
A typical SMTP exchange involves the sending server introducing itself, specifying the sender and recipient, and transmitting the message content. If the receiving server responds with a 4xx code — indicating a temporary issue like server load or a momentary hiccup — the sending server schedules another attempt rather than dropping the message.
Most mail servers use an increasing delay between each retry (commonly called exponential backoff), so the first retry might happen after a few minutes, with each subsequent attempt spaced further apart. This avoids overwhelming an already struggling server with repeated attempts in quick succession, while still giving the message a real chance to get through once conditions improve.
Common causes
- Server congestion: A receiving server handling a spike in traffic may temporarily defer new messages until its backlog clears.
- Greylisting: A common anti-spam technique where messages from unfamiliar senders are deliberately deferred the first time, on the assumption that legitimate servers will retry while spam senders typically won't.
- Network issues: Intermittent connectivity problems between the two servers can interrupt the exchange and trigger a temporary failure code.
- Misconfiguration: Incorrect DNS records or mail server settings can cause messages to be misrouted or rejected in ways that resemble a temporary issue.
Why persistent retries are worth investigating
A message that eventually goes through after a retry or two is normal and rarely worth worrying about. But if the same message keeps getting deferred across many attempts, that's usually a sign of something worth fixing — often a domain reputation issue or a misconfigured authentication record rather than a truly transient problem.
Reducing unnecessary delays
- Keep your mail server's resources (CPU, memory, bandwidth) appropriately sized for your sending volume.
- Review your retry and deferral timing settings if you manage your own mail server, rather than relying entirely on defaults.
- Monitor your mail queue and bounce rates regularly so a growing backlog gets caught early.
- Keep your SPF, DKIM, and DMARC records accurate and up to date, since authentication problems are a common hidden cause of repeated deferrals.
Common questions
How long will my mail server keep retrying before giving up? This varies by server configuration, but it's typically somewhere between 24 and 72 hours before a message is finally returned as undeliverable.
Is a deferral the same as a bounce? No. A deferral is temporary and the server will try again; a bounce (usually a 5xx code) is a permanent failure that won't resolve itself with more retries.
Can I speed up a deferred message manually? Not directly through the receiving server, but you can confirm your own configuration is correct so future attempts aren't deferred for reasons within your control.
