Mail 3.1 always forgets passwords
I have just switched to Mail on my new iMac after a year or two being largely satisfied with Entourage on my older iMac. I'm about to switch back. I have four accounts set up in Mail v.3.1 (the same ones that were set up in Entourage) but Mail continually forgets one or more passwords despite me checking the "remember password" box. It seems to happen to random mailboxes and at no particular time, but it frustrates the heck out of my wife and me. I've looked at the Keychain to see if it needed repairing, but it seems to be fine. The update to 10.5.1 hasn't helped. Does anyone have any last suggestions before I dump Mail and return to Entourage for good? Thanks.
iMac 2.8GHz Intel Core 2 Duo, Mac OS X (10.5.1)