Sadly, as the other commenter has pointed out, they rely on the recipient server understanding them. A lot of the fails I get are due to non-existent recipient

Sadly, as the other commenter has pointed out, they rely on the recipient server understanding them. A lot of the fails I get are due to non-existent recipients, full mailboxes and some that detect it as spam end up replying (to me) just to tell me that they've detected it as spam (which I know already, and would prefer they didn't aggravate the issue by replying). Bear in mind, I'm not talking about recieving spam directly (I get almost none) I'm talking about "back scatter" from innocent (but perhaps poorly configured) mail servers telling me they couldn't or wouldn't deliver "my" email. This all goes to randomly generated addresses at my domain (which the spammers using my domain are generating), which I receive at my catch-all and may have to consider disabling, while ensuring my mail server doesn't also notify them that it didn't deliver their bounce back (otherwise I'm perpetuating the issue) all at the risk of losing legitimate emails if they're miss-spelt etc.