To get an email for "domain.com" to be sent to email.domain.com, just do the following:
1) add and A record for email.domain.com.
2) change the MX record from domain.com 0 to email.domain.com 10
SMTP is both for receiving and sending mail at the server level. A mail server recieves mail from other servers using smtp, and sends mail to other servers using smtp.