New Post: SendReceipientMail Exception after upgrade from 1.2 to 1.5

Hi,

when I add some changes to an issue I am not receiving any notification emails any more.
The log shows the following entry:

System.Net.Mail.SmtpFailedRecipientException: Mailbox unavailable. The server response was: 5.7.1 Un...

Any Ideas?
Thanks in advance
Alex