I've seen people list email addresses at Gmail with "+" within the recipient name.
Here's an example of what I mean:
That said, if you attempted to create a Gmail account with "+" recipient name, you would get this error message:
Please use only letters (a-z), numbers, and periods.
Is the "+" simply read and removed by the mail server, and if so, is this specific to Gmail, or is it universal to all email addresses/servers? Meaning my guess is that people to this for readability, but I've always been puzzled by it, since it appears to not be possible to create an Gmail account using a "+".