A bounce is not an error page, it is a message from a mail server explaining why it did not deliver. The useful part is the code.

4xx: temporary

The sending server will try again by itself, usually for a day or more. Nothing needs doing. A deferral is also what our relay returns to the very first message from a brand new account, once, on purpose.

5xx: permanent

Nothing will be retried. The reason text matters:

• Alias disabled or inactive. The alias is switched off. Turn it on and ask the sender to send again.
• Unknown recipient. No such address here. A typo, or an alias that was forgotten and released rather than deleted.
• Over quota. The monthly forwarding allowance is used up, or, if you use the mailbox here, its storage is full.
• Message rejected by policy. A rule of yours blocked it, or our relay refused it as spam or malware. Blocked messages are counted on the alias, so a rising blocked figure with nothing arriving is a strong hint.

Bounces you receive rather than send

If somebody writes to your alias and gets a bounce, the text they see is the one we produced, and it is deliberately plain: it says the address is not accepting mail, and nothing about who owns it or why. An error message is a place where services leak more than they realise.

When we bounce instead of dropping

Where we can refuse a message at the connection, we do, so the sender learns immediately. Silently accepting and then discarding is worse for everybody: the sender believes it arrived, and you never learn that it did not.