The actual relevant question is: how many of you make a point of disallowing this? You have to go out of your way to make that not work, given that + is a valid character in email addresses.
Per another comment, all one would need for it to not work is have a regex of [a-zA-Z0-9]@[a-z.]+, or using POSIX-compat, [:alnum:].
* https://en.wikipedia.org/wiki/Regular_expression#Character_c...
Only a small minority of my email addresses I've ever had matches that pattern.
The example is illustrative, and not definitive, as to how the simple and innocuous decision about a regex can determine functionality, and it is not about someone going "out of your way to make that not work" to break things.
David.Calhoun@boeing.com
would be completely normal.
Yet the above regex would not match it. Whoops.
Likewise,
can_you_believe_it@hotmail.com
which is also completely normal and typical, would fail the regex as well.
But I guess if the only kind of customers you want are the
fred@hotstartup.com
and
jane@some-vc.com
people then sure, this otherwise wholly inadequate regex would serve you well.
And you can unintentionally cause sub-addressing to not work without having "to go out of your way to make that not work" like the GGGP wrote.