It appears that Sam Varshavchik <mrsam(_at_)courier-mta(_dot_)com> said:
My viewpoints are somewhat dated. The persona-non-grata status of MXs
pointing to CNAMEs – that verbiage appears to be new to the current SMTP
standard, I checked and did not find any equivalent language in the prior
one; so my implementation reflected that.
The author of 5321 is on this list so perhaps he recalls what the motivation
for the change was. I'm sure it wasn't accidental.
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