How to only receive mail?

I want to only receive mail on my server and not send it. What would be the best way to do this, block the outbound port?

The reason I want to do this is to prevent someone from getting on and using it for spam. I will do my best to keep all components updated, but things happen (like 0days).

For sending email, a password is required. So the risc of sending email (from outside) is limited.

If you create a descent password, it’s almost impossible to use it for spam.