Which is why you should have everything backed up using Outlook or similar.
Edit:
I couldn't think of the name of this earlier, but now I've remembered. I worked at a university and we used Pegasus Mail to send out mass mailings in our department. It worked great, and it's free. It looks like it has a good community behind it too; it's been around a long time. Check it out.
Good stuff-- credit where (absolutely due). My experience sounds very similar to OP.
The other software I remember fondly from that era was David Harris' Mercury and PegasusMail. I worked for a university (if you could call it that) and that was the mail server and client combo. Free (and it still is, apparently). I wrote a thanks-dude-you-rock e-mail to him once and he wrote something nice back. Nice Kiwi bloke.
Maybe try Pegasus Mail...used it years ago before moving to gmail (for personal mail) and Exchange (for work email). It hasn't been updated in quite a qhilw, but it was rock solid then, so I'm sure it is just as fast and stable if you just need POP3 or IMAP support.
EDIT - Make it portable
Yeah, definitely alarmist. Go ahead and email the developer (David Harris - and his email address is linked in the footer here) and ask him. I'll lay five bucks he gives absolutely zero shits.
Give credit where credit is due. Pegasus Mail first offered this feature (along with spam/content filters and other innovations). Sure, it's not pretty by today's standards, but the big things people laud about Gmail, Pmail users had been enjoying for years (at least those features that are possible for a local client rather than web service).
Pegasus 4.62 has lots of improvements and could be worth a look.
(It's freely downloadable although the author requests that you buy a manual to pay for it.)
Memecode has added IMAP to their free client i.Scribe (and also to their commercial client, InScribe). Also worth a look.