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You're right, it's not, but maybe people are wising up to the username+spam_source@gmail.com trick.

Wising up? Whenever I have an address like this rejected I usually attribute it to ignorance and an inability to read rfc5322.

If that were the case they'd accept it and just remove the +spam_source before selling the address.

Likely the email-checking regex just doesn't include that character

Exactly this. Just a few weeks ago, I noticed that this was the case for an application I'm maintaining at work.