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Lots of programmers haven't, though.

I get addresses rejected as invalid when signing up for some service at least once a month.

My favorite is one site I encountered that let you create and login with such an email address, but the forgot password form couldn’t handle it and would 500.

I've seen programmers get overruled by "product managers" in regards to handling email as it is intended to be handled.

A lot of services don't allow + in email addresses. With gmail you can also insert a . anywhere you like which works more often. But sometimes, catch all addresses really help to test.

What standard is foo+bar@test.com a part of?

I don't think there's a standard which says that "X+Y delivers to X" - it's a configurable option in exim and you could equally well make it "XqY delivers to X" if you were wilfully perverse.

It was already a known trick to identify sources of spam when I was student (1990-1993).

And spam filters