Thanks for the info. I've seen MailArchiva, and I am considering it also as an option for archiving our e-mail. I hadn't seen the detailed explanation of Journaling from Microsoft that you linked, so thank you for that, it's very informative!
Without actually trying anything it looks as though SME's "Keep a copy of all e-mails" function is very similar to Microsoft Exchange's Journaling feature.
Related to this, does anyone know of a way to copy mail only from or to specified users to the maillog (or another) account, rather than all mails on the server?