[ietf-dkim] Issue 1535 - clarify need for domain existence check in the decision tree (step 2)

[Frank Ellermann]

Steve Atkins wrote:

> As the hostname being checked is on the RHS of an email address  
> there must be an A record or MX record for it, in order for it to  
> be... operationally relevant, anyway.

JFTR, if all goes well 2821bis will explicitly add AAAA.  This does
not mean that "AAAA only" is an operationally good idea today, but
the "MX required" faction was hopelessly outnumbered for 2821bis.

 Frank