I know it has been talked a lot about the SMTP error in the email settings but I found out something very strange.
When I change the email settings or setting up a new one in the administrator area and send a test mail (via Exchange or via Other) WITHOUT saving this settings the test email was sent without any error and I received it.
When I do the same and SAVE the settings BEFORE I send the test email I get the smtp error.
So it cannot be that there is a misconfiguration or the port is not open.
Also Sendmail via a php script or via the console works fine.
I think there have to be a bug in the file include/SugarPHPMailer.php which is responsible (think so) for sending the mail.
I am not sure but the test mail without saving use the file modules/Emails/Email.php which is required once in the file /modules/EmailMan/testOutboundEmail.php
I really hope that this information might help someone in troubleshooting.
PHP Version 5.6.17-0+deb8u1
Server API CGI/FastCGI
Apache Version 2.4.10
MySQL Version 5.5.47