Greg Wooledge wrote: > My understanding: the SMTP receiver will use whatever heuristics it > finds appropriate to avoid receiving spam. > > One heuristic that is commonly used is to reject all messages where > the HELO doesn't even syntactically qualify as a valid FQDN -- in other > words, has no dot in it. > > Another heuristic that is commonly used is to perform a DNS query on > the HELO string, and reject it if it's not a valid FQDN based on DNS.
true and as I mentioned in one previous reply since couple of years every one is implementing DMARC and SPF. In near feature mails that are not signed, will not be processed/accepted or will score bad and be treated as spam.