Mail: Select SMTP server automatically on addressee
The automatic domain selection in Mail based on the address is not possible but....

There is a setting in Mail, send from the account of the selected mail box. 



Means, if you create or reply to emails while in the Mailbox of the University it will use that account. 

However, if you create reply from your other account it will use that one. 

So all you have to do with your 2 accounts is to make sure to use the right account when creating or replying.

Additionally you can use the "Mark addresses not ending with ....@ugent.be

Show email addresses that don’t end in a specified domain in red, to help prevent you from sending messages to unintended recipients.