mail.ietf.org. is ietf.org., so you can remove your MX records for
ietf.org. This should cut down on spam since a lot of spambots
will skip over domains whose MX list cannot be obtained. Real
mailers will of course fall back to A/AAAA as per RFC 2821/5321. A
few hosts will have trouble, but very, very few indeed, and that
isn't your (our?) fault.
I checked with some people who run mail for a lot of domains, and they
assure me that spambots will try to deliver to the A record even if
there is an MX. Where did you get the idea that not having an MX
offers protection from spambots?
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