I had the same problem .. after going through all the solutions stated here I still had this problem but have solved it now .. I was using Mail 5.2 ..
Please try this .. It solved my problem.
Go to Mail -> Preferences -> Accounts -> Account Information
on last option of Outgoing Mail Server (SMTP) click the drop down and click Edit smtp server List
Click Advance tab
uncheck 'use Secure Sockets Layers (SSL)'
make sure you have specified email address in text box for User name along with password
now click OK
Now on Advance tab (Mail -> Preferences -> Accounts -> Advance)
uncheck use SSL
Port should be 110 and authentication should be password
As there is no OK button you will be prompted to save changes when either closing this window or upon clicking on another tab on the top e.g Genral, RSS, Junk Mail, Fonts & Color etc
After doing the above I was able to receive my emails once again .. It seems this version of Mail is having problem with SSL