Email address validation takes place both on input by the browser and on submit by our code.

The problem is that there has been a change in our validation code for J4 that does not match the browser validation.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/email

A single properly-formed e-mail address. This doesn't necessarily mean the e-mail address exists, but it is at least formatted correctly. In simple terms, this means username@domain or username@domain.tld.

So if I enter into an email field in user management the browser will respond
brian This value is not valid
brian@example "no error message"
brian@example.com "no error message"

However the regex has changed (#19300 I think) and brian@example is rejected on submission

@wilsonge this is a summary of what I mentioned