[sylware]

I 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.

[mtmail]

Do many websites support email address with ip literals? john@12.34.56.78 ? I haven't seen such email addresses in years.

[sylware]

Any competent sysadmin will handle those, with serious software, ofc.

That said, this standard format is:

mailbox@[x.x.x.x] and mailbox@[ipv6:...]

[sylware]

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.