Issues with the Default Email address for bounces - authentication
CiviMail doesn't concatenate the username to the servername, it just takes whatever you provide and tries to connect using those credentials.

But my guess is that the issue is unrelated to the username, since your error message is:

Failed to connect to the server: mail.mbrainingcentrum.com:993.


I'd guess that you've more likely got a firewall issue, or an SSL issue on your server.

You can test by manually trying to connect to your mail server, using e.g.  the instructions here: 
https://technet.microsoft.com/en-us/library/aa995718(v=exchg.65).aspx

[As per comment below - another thing to check is that the server is connecting to the correct machine, e.g. if it has a local dns setting that is wrong.]