There are email rules and GMail rules. RFC 822 was the original spec for email as we know it. I'm not going to look them up because I don't have time. Case doesn't matter. You can use what you want and anyone else can too. GMail ignores ".". You can scatter them willy-nilly in your name and get the same account. This is not sanctioned by RFC 822 (for both senses of the word). "Normal" mail systems have typically ignored everything after "+". Probably not GMail. I sometime use that to put tags on email addresses I give out: I can tell who gave away my email address to a third party. Unfortunately, many web sites refuse "+" within email addresses, clearly violating the RFCs. I have inferred that this is a Microsoft innovation.