Because of security reasons, we block port 25 (SMTP) for outgoing traffic.
You can't change that. However, it is possible to use port 465 or port 587 instead. It is also possible to use an external mail gateway to send traffic out from your mail server.
Incoming mail using port 25 is not a problem and is allowed.
You can always check with our sister company SQR.nl for (email) hosting solutions.
We have a small news item on this subject (2017), if you want to know why we are forced to block port 25.
Or maybe you were looking for a related question: Which ports are open/closed in the default security group?