"Andrew G. Tereschenko" <spf-discuss(_at_)spam(_dot_)24(_dot_)odessa(_dot_)ua>
writes:
Do SPF restict all "pass" email addresses to have valid MX ?
If yep - so why the we need "+mx" directive ?
I do not see any reason to block emails comming from limited number of IPs
that listed as MX for domain.
Probably "+mx" was added becouse we would like to avoid useless MX record
lookups.
Consider the case of a secondary MX. This will often (especially if
run by someone other than the domain owner) only be used for receiving
emails never for sending them. So, in such cases a +mx would not be
appropriate.