could be the ATT server, I have both my .mac mail and my comcast.net mail accounts in my Mail.app and it periodically asks me for the Comcast one and sometime even the .mac one as well so it's probably nothing to worry about.
oh and in the future post threads like this in the Mac Help section.....thanks.
It is definitely the ATT server. I too have ATT Cable and it has done that off and on in the last couple months. They have been having problems with there system. If there was a competitor here in my area I would have them in a heart beat.
Next time it happens take a look at your Cable modem lites. Are they out or blinking synchronously? If they are you cable modem is having problems connecting and quite often unplugging it or turning it off for a few seconds then back on will allow it to reconnect. It appears it's not necessarily a modem problem but some sort of glitch in there system. It was doing the login in thing to me not stop for a couple weeks and I was extremely irritated but just about the time I was ready to call about it it went away and hasn't done it for a month now.
it's your mail server. my university mail account is always crapping out and mail asks me for the password, or says that it cannot contact the server. there's really not much you can do about it as the problem is on the other side of the pipe.
Yeah, Mail ask for my password and then reject it constantly if there's a connection problem with the server. I think it equates password rejections with connection timeouts or some other errors. For me it almost always happens when i'm using all of my bandwidth, like having several downloads and uploads going at once...