Mail keeps forgetting passwords

I was having a problem with Mail forgetting my password for my email account and even though I woulkd keep renetering it, Mail refused to accpet it. I installed the latest update for OSX Snow Leopard and that seemed to take care of it. I also reset all of the Keychains for each email account and reentered my passwords. Now Mail is again refusing to accept my password on some of my email accounts. This is getting to be rediculous. I noticed that some of the other applications such as Contacts and the Calendar also have problems linking to my email addresses when I open them.

Is Keychain getting corrupted? or what is going on?