[sylware]

Many email servers do forget about the email adresses with IP literals, that for people who are self-hosted without paying for DNS.

mailbox@[x.x.x.x] and mailbox@[ipv6:...] (and probably without "ipv6" prefix once ipv4 is gone).

This is stronger than SPF since the second the IP of the sending SMTP server does not match the IP in the "from" headers and the envelope, the email is dropped, not even going into spam.

For instance, currently, if I send an email to a gmail slave, their parsers will ask for... a DNS PTR record, Oo "Geniuses" at work, or conveniently breaking all interop with small tech?