This comment was added as a follow-on note. Sorry for not being clear.
Accepting messages from a domain lacking MX records might be risky due to the high rate of domain turnovers. Within a few weeks, more than the number of existing domains will have been added and deleted by then. Spammers take advantage of this flux. Unfortunately SMTP server discovery via A records is permitted and should be deprecated.
All it would require is a couple of large ISP's to adopt such a policy. "MX 0 <self>" really is not hard and benefits the remote caches.
Once MX records are adopted as an _acceptance_ requisite, domains not intended to receive or send email would be clearly denoted by the absence of MX records. SMTP policy published adjacent to MX records also eliminates a need for email policy "discovery" as well. Another looming problem.
Better yet us MX records to signal that you don't want to receive email e.g. "MX 0 .". It has a additional benefits in that it is *much* smaller to cache than a negative response. It's also smaller to cache than a A record. Since all valid email domains are required to have a working postmaster you can safely drop any email from such domains.
Don't accept a message from a domain without MX records. When there is no policy record adjacent to the MX record, there is no policy, and don't go looking.
-Doug