SMTP or X.400

I have 3 E2K Servers connected in my domain.
Server 1 & 2 is in one site and can both send mail to server 3 in remote location.
I would like to know when E2K send mail over SMTP and when X.400 is used.
I recently had problems with Q getting backed up with mail on the SMTP side.
I also saw some mail (SMTP type) in the X.400 Q’s.

Can anyone HELP ?