I have SA installed on our 5.5U3. I run spamd at startup, and procmail calls spamc to check each email. I made a custom template so that the SA check is automatically added when a user switches on procmail (via User Panel).
/etc/e-smith/templates-user-custom/.procmailrc/39spamassassin:
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
# --------------------------
# SpamAssassin spam filter
# --------------------------
:0 Wc
| spamc -c
:0 We
$MAILDIR/Spam/
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
This dumps the bad emails into a "Spam" subfolder, which is a regular IMAP folder that users can subscribe to.
SA can be setup in so many ways it's hard to answer your question. I'm not sure how the contrib is setup by default. The file listing from "rpm -ql" might give you clues about where to look for stuff. You can have SA just add headers stating that it was detected as spam, or you can have it modify the subject line, or you can use the check results to process it via procmail. If you just want more info on how SA works in general, check its site at
http://www.spamassassin.org and look at the docs.