[farfatched]

Could they have consciously chosen to remove that functionality?

E.g. to simplify code, or if they wanted all mails to have a domain (if, for example, they wanted to integrate with reputation systems that were domain oriented)?

[jeffbee]

Based on the incredibly basic bounce message, I suspect the problem is that the frontend eats the address before it even gets to delivery.

To your question, yes any product decision is possible, but enterprise/government people are surprisingly demanding about this stuff working because they have extremely weird requirements for routing mail to and through legacy systems. So I bet this still works at the mailer level and is broken in the UI.

[jcranmer]

I chuck IP address literals (both IPv4 and IPv6) on the list of things that you should care about for email if you're writing an MTA or an MUA but should otherwise generally not care about supporting if you're using email for something else (e.g., as a UID for login).

[farfatched]

> but enterprise/government people are surprisingly demanding about this stuff working because they have extremely weird requirements for routing mail to and through legacy systems. So I bet this still works at the mailer level and is broken in the UI

Interesting context, thanks.