Vanishing password in Mail

On my Macbook, I use Mail when on the road. Lately, every time I open Mail it asks me for my password. I supply it and compose a message. When I try to send the message, it asks me for my password again. In Mail Preferences, I type in my password for the account, click on the "save changes" box, and close Preferences. When I reopen preferences again, the password is missing. Also when I am asked for the password in order to send or receive messages, I fill in the password and click on "save password in keychain" box. No change - it continues to ask for the password every time. Any help in solving this annoying problem would be appreciated.