In general we will never know it (we get no bounce mail or anything) -

If you're not getting a "bounce" there's something wrong with how you're
sending the email. Well...actually, that's less true today. I'll explain
why.

Once upon a time, the blocking tools ran at the server level. That is, the
mail server to which your mail server was trying to send the message would
reject it. The failed message would still be on your server, and it would
be your server's responsibility to send the "bounce".

However, users have - justifiably - become concerned about "false
positives". So the model has been changing. Instead of servers rejecting
email, the mail is now delivered but into a special folder. The user can
ignore this folder, scan it occasionally, delete it, or anything in
between.

Unfortunately, though, this means that the mail was accepted by the
destination server. This has a number of problems, but one of them is the
lack of an error message.

It's tempting for some to blame the users for this, in that they're the
proximate cause. However, the blame truly lies with the spammers. W/o
them, the problem simply wouldn't exist.

[...]
Royal pain, and a cure as bad or worse than the disease.

It is a pain, but most people disagree with your assessment. They'd rather
not be buried in spam. If you need to do some extra work as a result,
those people don't care.

It's just aother aspect of the cost-shifting nature of spam. The spammers
spam, and you pay the cost. That's part of why so many consider it
"theft".

- Andrew