Many will reject your messages. Exim says something like this:
[quote:f2aec7790d]** [email protected]
.mx: all relevant MX records point to non-existent hosts: it appears that the DNS operator for this domain has installed an invalid MX record with an IP address instead of a domain name on the right hand side[/quote:f2aec7790d]
The vast majority of cpanel boxes will have lines like that in their exim_mainlog files (as well as many non cpanel boxes, of course).