On Sat, Jul 19, 2014 at 10:38:14AM -0400, Dave Crocker wrote: > >> No, it designates non-/receiving/ domains: > > > > That's a bit of an under-statement, because, for example, > > with the widely practived: > > > > reject_unknown_sender_domain > > > > restriction in Postfix, mail from nullmx domains *is* rejected, > > so any that declare-themselves non-receiving become non-sending > > in practice. If the draft suggests that nullmx domains might > > expect to send mail will impunity, that would be misleading. > > > > In practical terms, if you don't receive, you can't send (that is > > you can't use the domain in the SMTP return path). > > > Sorry, no. > > 1. The return-path is not required to have any relationship to any > another email field. > > 2. The semantics of nullmx make an assertion only about receiving. > > In fact, there are legitimate scenarios that include use of domains used > for sending but not receiving, as the draft cites. > > The fact that some operational choices might not allow this scenario is > a separate matter, outside of IETF specifications. One can bury one's head in the sand if one likes, but non-receiving domains that turn on nullmx (or whatever it ends up being called) need to be aware that in practice they will find their mail rejected by many receiving systems if they attempt to use such a domain in the "MAIL FROM:<return-path>". -- Viktor.