What is a retry schedule?

A retry schedule is how long a mail server keeps trying to deliver a message that got temporarily rejected. If your email bounces with a "mailbox full" or "server busy" error, the sending server doesn't give up immediately. It waits a bit, tries again. Then waits longer. Tries again. Eventually it either succeeds or gives up and sends you a bounce notification.

Most mail servers use exponential backoff. First retry after 5 minutes. Next after 15 minutes. Then an hour. Then 4 hours. The timing varies by server, but the pattern is the same: try fast at first (maybe the issue clears up quickly), then slow down (don't hammer a server that's actually down). Postmark retries for 45 hours before giving up. SendGrid retries for 72 hours. Your own mail server's default is probably somewhere in that range.

Why this matters to you: if a recipient's mailbox is full on Monday morning, your email might still get through Monday afternoon after they clear space. But if their mailbox is full for three days straight, the retry schedule expires and your message bounces permanently. You'll get a Non-Delivery Report (NDR) saying the server gave up.

The retry schedule is not something you typically configure unless you're running your own mail server. Most ESPs handle this automatically. What you DO control is what happens after the retries expire. Do you suppress that address? Try again next campaign? That's on you.

One thing to watch: if you see the same addresses bouncing with temporary errors every single send, that's not a retry schedule issue. That's either a permanent problem (mailbox quota they never fix) or a reputation issue (the receiving server is soft-bouncing you because it doesn't trust your domain yet). Check your sender reputation before blaming the retry logic.

If you want to see retry attempts in action, check the email headers next time you get a delayed delivery. You'll see multiple "Received" stamps showing each retry attempt. Or use our free header analyzer to decode them.