Uncomment this line and check (in the logs, when sending test email) that the values are correct just before being sent to the server:
https://github.com/salesagility/SuiteCRM/blob/master/include/SugarPHPMailer.php#L457
Next, uncomment these lines to get an extensive log of your SMTP interaction with the server. It should provide you with an exact error.
https://github.com/salesagility/SuiteCRM/blob/master/include/SugarPHPMailer.php#L495-L502