Veeam to permit traffic from it to the server on port 25
Additionally, add 465 port if you are connecting to smtp server ssl. The good way to check if it is connectivity issue –to run telnet from your Veeam server to mail server(By default telnet client is off in Win7, so you would need to enable it in Programs > Turn Windows features on or off > Telnet client).
To run telnet test go to Run >cmd
and type telnet yoursmtpserver 25
to check connectivity for SMTP or telnet yoursmtpserver 465
to check connectivity for SMTP over SSL and see the result.