I manage a few personal mail relays that I use for relaying my own mail and for experimentation purposes (mail logs are a great source of unique and continuously flowing data that can you use to try out different ideas in GUI, database, or parser development). One of them was acting up recently. I got a message from my upstream mail-queue host saying that they’ve queued up quite a bit of mail for me over the last few weeks, and that I should investigate, as they do want to avoid purging the queue of valid mail.
Clearly I wanted to avoid queuing up mail on a remote server that is intended for my domain, and so I set out about understanding the problem.
What I found was that there was a setting in my /etc/postfix/main.cf that, although it was technically a valid setting, was incorrect for the role that mail-server was playing. Specifically the mail server was supposed to be rejecting email completely for non-local users, instead of just deferring it with a “try again later” message.
In this case, I’m using Postfix v2.5.5. The settings that control this configuration in /etc/postfix/main.cf are as follows:
local_receipient_maps defines the accounts that this mail server will accept and relay mail for. All other accounts would be “rejected” by the mail server.
However, how rejected mail is treated by Postfix depends on how it is configured, and this was the problem with this particular server.
For Postfix, it is possible to mark a message as “rejected”, but actually have it mean “rejected right now, but maybe not permanently, so try again later”. This “try again later” will cause the e-mail message to be queued on the upstream server, until it reaches some kind of retry time-out and delivery is once again attempted. Of course this will fail again, and again.
This kind of configuration is great for testing purposes, because it allows you to test the same messages over and over again without losing them, or to queue them up so that they can be reviewed to ensure they are indeed invalid e-mail messages. However this is not the state you want your mail server to be in permanently. At some point once things are ready for long-term (production) use, you want your mail server to actually reject messages permanently.
That is where unknown_local_recipient_reject_code comes in. This configuration property controls what the server means when it “rejects” a message. Does it mean right now, or permanently?
The SMTP server response code to reject mail permanently is 550, and the code to reject mail only temporarily is 450.
Here is how you would configure Postfix to reject mail only temporarily:
unknown_local_recipient_reject_code = 450
And here is how you set Postfix to reject mail permanently:
unknown_local_recipient_reject_code = 550
In my case, changing the unknown_local_recipient_reject_code from 450 to 550 is what solved the problem.
In summary, if you ever run into an issue with your Postfix mail server where you believe mail is set to be REJECTED but it still seems to be queuing up on your up-stream mail relay, double-check the unknown_local_recipient_reject_code.
# Local recipients defined by local unix accounts and aliases only local_recipient_maps = proxy:unix:passwd.byname $alias_maps # 450 (try again later), 550 (reject mail) unknown_local_recipient_reject_code = 550