If a message comes in incorrectly addressed, yes, it will bounce. It should, shouldn't it?
Yes, but only by ejecting the message immediately during the SMTP session. Accepting the message, then realize it can't be delivered and sending a bounce message back to the email address listed in the "from" header is NOT the right thing to do for reasons that should be apparent by now.
I actually think that the spamassasin/procmail combination above is nearly ideal on the MUA side,
It is not, because:
1. Bandwidth is used up by spam (which is fortunately usually not that big) and worms (which tend to be much bigger)
2. A lot of processing time is used on your system(s)
3. Either:
3a. Legitimate senders who are tagged as spam are blackholed and are unaware that you don't read their message
3b. You must manually sort through all messages that are flagged as spam