Spam is usually dealt with at the MTA level. And, honestly, I usually don't deal with that sort of thing (my personal feeling is that one mistakenly flagged message isn't worth any number of correctly flagged ones). However, I cannot imagine anything that procmail could do that Sieve couldn't (but I'm not an expert at either one, so I could be wrong).

Well I agree with the flagged part, but I have my spam configuration set to not delete any messages, nor reply automaticially. Basicially the Spambouncer package assigns a score to the e-mail. The higher the score, the more likely it's spam. So I have 4 total basic folders, Inbox, bulk, blocked and spam. Spam is the highest scoring, and usually I get hit with 1-3 a day here. Out of all the messages that go there, very few are actually legitimate. Blocked is where lower scored ones go, and I get about 20 or so new messages there. These do contain a legitimate message here and there, but still a small percentage. Bulk gets any mail not directly to me (based on putting all my addresses into the config), and Inbox gets the rest. It works out well, because I don't miss peoples e-mail even if they do get filtered wrongly. I just tend to visit the spam boxes a few times a week, scan the subjects for anything I am expecting, and trash the rest.

Messages that are legitimate get filtered before the scoring system, saving both CPU time and my time since they typicially go to their own box. (CPU time due to the massive amount of filters, and DNS lookups it does against blacklist servers)

Replacing procmail with sieve would be a pain, because the Spambouncer is 477k worth of procmail scripts, typicially updated at least once a month.

I'll play around with the IMAP server you are recommending on a spare machine. I just don't see the benefits outweighing the work involved, considering it still dosen't solve my desire to archive mail easially either.