What’s the difference between 421 and 451 errors?

You're checking your delivery logs and you see a mix of 421 and 451 errors alongside otherwise successful sends. Both are temporary, both cause retries, but they're telling you different things.

421 means the server is closing the connection. It's an abrupt "we're done here." Common causes: the receiving server is overloaded or under maintenance, you've hit a connection-rate limit, or the server detected something it didn't like and decided to end the session. It's often a server-side issue rather than anything specific to your message.

451 means the server accepted the connection but couldn't process the message. The reasons vary more widely: temporary authentication failures, greylisting, a transient policy decision, or the server's own internal queuing problem. A 451 often comes with a longer explanation in the response text, which is worth reading. Both codes use proper backoff and retry logic in well-configured sending systems. Your MTA or ESP should retry these automatically at increasing intervals. If a 421 or 451 persists across many retries without resolving, that's when it stops being a temporary glitch and starts being a signal that something's wrong with your reputation or configuration.

One thing to watch: if you're seeing a lot of 421s from a specific provider like Gmail or Outlook, it usually means you're sending too fast for your current reputation level. Pull back volume to that destination and see if the errors resolve over a day or two.