[centur]

IIRC dot is one of the characters that can't be discarded when checking local addr part (RFC 5322). So [e-mail] and [e-mail] are different addresses really. As far as I understand - it's a Gmail's team decision to configure local addr interpretation and allow `[e-mail]` and `[e-mail]` to be treated as the same address. I'd expect that dot trick rarely works anywhere outside of gmail world.

+ sign is part of the standard (`atext` token, RFC 5322), so sites, which disallow it in address are doing it wrong. The fact, that industry adopted a practice of using everything after + sign as a "tag" is not captured anywhere so this creates even more mess in already messy space (e.g MS followed GSuite in this too and added subaddressing - https://learn.microsoft.com/en-us/exchange/recipients-in-exc...)