Can i run smtp server and have MX simultaneously?

I had an MX set up for GMail for my domain. I also have an email sent out automatically whenever someone creates a new user on my site with the following settings:

:address => “”,
:domain => “”,
:user_name => "",
:authentication => :login

This worked before I set up MX, and after I disabled MX, so I’m guessing I can’t have MX and smtp at the same time. Please is there some way to make this work? Thanks very much.