SMTP Relay

I'm confused a little bit, if someone uses SMTP relay to send emails what they will use to receive emails?

Ok, since there are 74 views for this thread (so far) and no one answered so i guess no one knows the answer...

I have the answer now, in order to receive email, you should have a domain registered and have an MX record pointing to that domain mail system IP (or you IP)..
Hope it is clear now.