[pagekalisedown]

You're right, it's not, but maybe people are wising up to the [e-mail] trick.

[dfc]

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

[Splognosticus]

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

[artursapek]

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

[brazzy]

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