If you uncomment this line:
https://github.com/salesagility/SuiteCRM/blob/master/include/SugarPHPMailer.php#L457
you will see more information in your logs (st them to DEBUG level in Admin/System settings).
And also here
https://github.com/salesagility/SuiteCRM/blob/master/include/SugarPHPMailer.php#L497-L502
if you un-comment that, you will get a full SMTP log in suitecrm.log. You can see what the email server is complaining about.