> Yes. Any email service should allow it, as it's standardized and very old.
As far as I know it’s not standardized at all; it’s not in the SMTP spec. It’s actually annoying that some people assume this is standard because they can break functionality, like assuming foo+bar@ is the same person as foo+qux@ when it’s not guaranteed to be.
It's not in the SMTP spec because SMTP doesn't need to know about it. Your objection is correct but in fact that's the desireable behavior -- senders can't assume that foo+bar and foo+qux are the same person. If they could make that assumption, subaddressing would be defeated as a useful filter signal. As it is, it's not just plus signs -- qmail has been using '-' as the subadress separator for decades.
Subaddressing is standardized in RFC 5233 as a filtering signal.
As far as I know it’s not standardized at all; it’s not in the SMTP spec. It’s actually annoying that some people assume this is standard because they can break functionality, like assuming foo+bar@ is the same person as foo+qux@ when it’s not guaranteed to be.