Hotels in Europe to sue Booking.com over 'abusive' practices
theguardian.comI don't think booking.com has still noscript/basic (x)html interop supporting email address with ip literals (for self-hosted people).
If the case, this dominant online service should be regulated(Trump like? namely tariff?) to restore such interop.
Do many websites support email address with ip literals? john@12.34.56.78 ? I haven't seen such email addresses in years.
Any competent sysadmin will handle those, with serious software, ofc.
That said, this standard format is:
mailbox@[x.x.x.x] and mailbox@[ipv6:...]
I forgot: for obvious reasons, those emails are stronger than SPF since there is a direct correlation between the email address and the smtp server.