Another Mac Mail problem

So I was having some email correspondence with a colleague via email, in the middle of our conversation after several responses it suddenly tells me Cannot send message using server My Mail. When I go to edit SMTP everything seems in order, and I am still recieving messages and can send them out with another email server.

The only thing I find odd is that in Preferences; Accounts; advanced. My mail has a different port, SSL is not checked, and the authentication is set to password.

On the one that works a different port is used and SSL is checked. Would this make a difference? And why would it suddenly not work after sending two or three messages already?