I think it is critical to know which problem your SMTP is encountering.
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#L444
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#L483-L490