said by wayjac:
I think there are too many email clients set to automatically check for email every 30 minutes or less
This, and one other issue it could be. There could be a captcha on the account that needs to be cleared. Go to your webmail via a non ATT branded URL (mail.yahoo.com) and log into your webmail from there. If you get a captcha, clear that and sign out of webmail and try your client mail again. This also clears some up.