What is a retry policy?

You hit send, but the receiving server is temporarily unavailable. Does your system give up immediately? Try once and bounce? Keep trying for a week? That decision lives in your retry policy.

A retry policy is the set of rules your MTA (Mail Transfer Agent) follows when an email can't be delivered on the first attempt. The MTA is the software that actually routes and delivers your outgoing email, think of it as the postal worker handling your messages once you hand them off. The retry policy tells that postal worker how long to keep knocking before returning the letter.

Three things a retry policy defines:

• Queue lifetime. The maximum time a message stays in the outbound queue before the MTA gives up and bounces it back as undeliverable. The standard is 5 days, per RFC 5321, though time-sensitive systems often use shorter windows.
• Retry intervals. How frequently the MTA attempts redelivery. A good policy starts aggressive (every 5 to 15 minutes) and then backs off gradually (every 1 to 4 hours). This pattern is called exponential backoff. It avoids hammering a struggling server while still giving delivery a real chance.
• Maximum attempts. Some policies cap the total number of retries regardless of how much time has passed.

If you're running Postmark or a self-hosted MTA like Postfix, you'll recognize these config settings:

• maximal_queue_lifetime = 5d
• minimal_backoff_time = 300s
• maximal_backoff_time = 4000s

Getting the balance right matters. A retry policy that's too aggressive can get your IP flagged by recipient servers for flooding them with repeated connection attempts. One that's too conservative might bounce legitimate emails that were only deferred temporarily because of a brief outage on the other end.

Most production setups land somewhere in the middle: start retrying quickly, slow down over time, and give up after 3 to 5 days. If you're seeing a lot of delayed or bounced messages and can't tell if it's a retry issue or something else, our free Email Header Analyzer can help you read what actually happened during delivery. Or if things feel broken, the SOS hotline is there (and it's free).