That + sign as a label to the same inbox is a Gmail thing.
It's a 100% valid character to use. Doesn't have to mean it's a label in another hostname. Goes back to what others have said. You either end up with a ton of rules, or go with the fact that email isn't the best solution to identify unique users.
> The local-part of the email address may be unquoted or may be enclosed in quotation marks.
>If unquoted, it may use any of these ASCII characters:
> - uppercase and lowercase Latin letters A to Z and a to z
> - digits 0 to 9
> - printable characters !#$%&'*+-/=?^_`{|}~
> - dot ., provided that it is not the first or last character and provided also that it does not appear consecutively (e.g., John..Doe@example.com is not allowed).[5]
> If quoted, it may contain Space, Horizontal Tab (HT), any ASCII graphic except Backslash and Quote and a quoted-pair consisting of a Backslash followed by HT, Space or any ASCII graphic; it may also be split between lines anywhere that HT or Space appears. In contrast to unquoted local-parts, the addresses ".John.Doe"@example.com, "John.Doe."@example.com and "John..Doe"@example.com are allowed.
I know that it's allowed; but — given that we're a B2B company serving highly-technically-literate customers — I don't think I want the business of anyone who thinks it'd be a good idea to use it even when it is allowed, given how it'd affect their own deliverability to imperfectly-implemented MTAs.
(It's a similar "you're really relying on perfect competence from the whole rest of the ecosystem to get you out of this" feeling as e.g. putting a space in your user name — and thus your home directory path — in Linux. Sure, the core GNU/BSD/etc tooling has been tested for that use-case — but are you really going to trust random tools and shell-scripts to handle argument tokenization perfectly? Or are you just going to ditch the space to be safe?)
The more technical literate a user is, the more they'll understand and figure out how things work, the nuances, and use those things.
You know that core tooling that works? It's because technical literate users, tried it, found the issue, and fixed it.
Same way with me using é on my name. If something breaks on a website, email, etc, I create an issue and starts emailing. My name doesn't have an e, it has an é.
It's a 100% valid character to use. Doesn't have to mean it's a label in another hostname. Goes back to what others have said. You either end up with a ton of rules, or go with the fact that email isn't the best solution to identify unique users.
https://en.wikipedia.org/wiki/Email_address#Local-part
> The local-part of the email address may be unquoted or may be enclosed in quotation marks.
>If unquoted, it may use any of these ASCII characters:
> - uppercase and lowercase Latin letters A to Z and a to z
> - digits 0 to 9
> - printable characters !#$%&'*+-/=?^_`{|}~
> - dot ., provided that it is not the first or last character and provided also that it does not appear consecutively (e.g., John..Doe@example.com is not allowed).[5]
> If quoted, it may contain Space, Horizontal Tab (HT), any ASCII graphic except Backslash and Quote and a quoted-pair consisting of a Backslash followed by HT, Space or any ASCII graphic; it may also be split between lines anywhere that HT or Space appears. In contrast to unquoted local-parts, the addresses ".John.Doe"@example.com, "John.Doe."@example.com and "John..Doe"@example.com are allowed.
(it keeps going on the wikipedia page)