I have a Postfix mail server with a catchall account. I had this catchall forwarding to another account on a separate machine/domain. I had to stop that, because I have a mail relay service in place (i.e MailGun) and the result of forwarding spam received by the catchall ran up gigantic, painful bills!
I want to just inform the senders of valid emails that get caught in the catchall now that the account they are trying to use is not valid. I figured a bounce back or an auto reply would do the trick. Then, I realized that an auto reply at least would be just like my original forwarding situation - it would end up involving the for pay relay. So that's no good!
Would a "bounce back" do that too? An implicit or explicit one? Is there a good (and relatively painless) solution aside from just "swallowing" this unwanted spam?
SMTP has a built-in mechanism for informing the sender that there is no such account available when sending an e-mail to non-existing accounts. One does not need to set up a catch-all e-mail address and auto-reply for that purpose. I guess this is what you meant by "bounce back".
The only benefit for the auto-reply is that one can make the error message more friendly, since the SMTP error message isn't that end-user friendly.