Hi and welcome, zon,
I'm sure there'll be better answers along before too long, but the first question (possibly way off the mark) that occurred to me is, do you use a router which might play with your server's IP address? Since I don't use one myself, I haven't a clue whether avast can handle those as neatly as with "ordinary" connections.
My primary mail is with Eudora (6.2) off my ISP's server, and I had no problems at all, either with the Mail Wizard under 4.1 or with the new setup under 4.5.
Best,
Mike