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TLDR: CNAME doesn't just forward A records, but MX records and others as well.

Just verified that this is not the case with dns records done through the domain manager at mediatemple.net

May be the case elsewhere (just no time right now to test this).

I would have started this reply by saying "not true" but don't know if the particular dns manager at mediatemple.net is setup in a way to be not compliant with the RFC.

Update: Upon further testing it does return a MX record but can't resolve the host name for the MX record.

It's really not a good idea. The issue isn't just whether mediatemple lets you do it... it's whether every DNS server that needs to talk to your domain understands it (and they don't).

Then couldn't you then add your MX records under 'www' and all will be well? Does this affect DKIM?

Except if you add the MX records with a hostname of "www", they'll be used for emails to "your_name@www.example.org", which is generally not the desired behavior.

And that still doesn't solve the issue with SOA/NS records, which need to exist at the root and thus will still conflict with the CNAME.

It shouldn't affect DKIM as that gives a specific subdomain, which is always prepended to the root domain to get the key.

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