Failed to connect to the server relay.skynet.be

I came across this discussion with the same problem, and after trying hard and using the diagnostic tool in Mail.app, I finally found the solution.

Mail.app tries on its own, without being asked, to perform a secure connection or authentication is requested, and then it crashes.

The thing is, in the SMTP server configuration section, set it to relay.skynet.be, no password (authentication method: None) and force it to use port 25, so not the section where it offers 25 and two other ports, but use the part where you have to enter the port yourself in the editing box.