New convert to Mac from years of Windows. Love it apart from the Mail. Use either iMail or Thunderbird but have the same problem. Using Lion. BT Internet mail account. Set up account in both programs using email address and password and works with no problems. Shut down. Start up next day and open either program and it asks for password. Put in password and it won't accept it. In the end, I delete the account and then go to Yahoo and change my password. Back to mail, set up with account and new password and it works again. Until the next day when we do the same again. This is driving me mad! I have read the forums, Googled it and looked at so many threads but nothing seems to work. Thanks for any suggestions.