I am getting a mail server fault MFRM53 Relay denied when sending an attachment

Lets look at this. Each mail identity has an outgoing mail server in outgoing servers(SMTP) in account settings associated with it. I suggest you check the server being used when you send mail is the correct one. That can be found in the identity part of the incoming mail account (pane above server settings) Next check the settings are correct as per your mail provider. That error specifically says "Relay Denied-unauthoruzed." so check that your outgoing servers all are set to use authentication. Unauthenticated sending of mail is something from the last century really. It is occasionally still seen in corporate environments where access control is sometimes as much physical as it is software but generally i is most uncommon. So to summarize; # Check you are using the SMTP server for the email address. For instance if sending from a yahoo address you need to be using the appropriate yahoo ongoing server using the correct user name and password for that email address. # Check the account settings for the SMTP server match those published by your mail provider (unnamed here) sending mail is a process of connecting to the correct outgoing server with the correct settings provided by the mail provider.