Some mail services allow a user to append a tag to their email address (e.g., where firstname.lastname@example.org is the main address, which would also accept mail for email@example.com or firstname.lastname@example.org). The text of tag may be used to apply filtering and to create single-use addresses.
I see developers get this one wrong all the time. And, actually, they don’t get it wrong: they copy someone else’s regex or validation method, and never bother to test it. If I’m filling out a form online, I’m probably going to give you my Gmail address, and I’m probably going to inject “+yoursite” before the “@” symbol. That helps me track who’s abusing my trust.
But: you have a requirement to “reject” invalid email addresses. What’s a valid email address? Well, to get completely pedantic, lots of things are technically valid email addresses that you’ll never run into. This is one of those cases where I’d actually rather see a set of tests — good emails and bad emails — rather than a “requirement” (“email should be valid”) or the specification itself. (Are you doing anything useful with that email address? Are you confirming that it belongs to the user? Because if you can’t answer those questions, a vague requirement about “valid” emails is just a distraction.)
Truly, the requirement should be based on the audience and expected input. Are you creating a site where people might be typing in their username rather than their email address? (Do your users confuse their mailing address with their email address?) Protect yourself against the scenarios you’re likely to run into, and then pour your energy into other ways to make your form better. (Not writing the fix for the edge case that means
an.”unusual\ example”@clownpenis.fart doesn’t pass, because I guess it’s a valid email address.)