I have a center where two employees are using Thunderbird and they are the only ones getting this message. It just started today. No changes were made. The error is- An error occurred while sending mail: The mail server responded: 5.1.2 The address specified is not a valid RFC-5321 address. h81sm2028729ywc.40 - gsmtp. I have verified all setting and verified the email address they are attempting to send to. They are able to recieve incoming mail but cannot reply. If they use gmail they can send and reply just fine. Others in the center are using outlook at they are able to send just fine.