If there is no MX record, the mailer will attempt to establish contact with the machine whose ip is returned through the cname lookup. If there is no matching mail account on that machine, the mail will bounce as there is no published route to the domain in question.
If you have MX records setup, then mail will go to the first available server based on priority level of the MX record. That being said, if there is no MX record for CNAME.domain.name then the mail will still bounce if no mail server resides on said box, or if no equivalent mail account exists on that box.
(if
user@CNAME.domain.name doesn't exist, the mail will bounce, unless there is a catchall address set up on that server.)
Many times, the default mail handling has a catchall the redirects catchall mail to /dev/null, effectively making it dissappear into the subetha.
