64 max user
I can't seem to find this anywhere.
Fortunately, that RFC is written in English, so you don't have to worry about learning Greek. Seriously, though, that RFC has actually been superceded anyway, by RFC 2821. Nonetheless, I won't make you read that, because that's a lot of reading to do to find this info.
In the context of Exchange, an SMTP recipient address consists of a display name (up to 55 alphanumeric characters) and the following required information:
1. Recipient/mailbox name (1 through 63 characters)
2. Domain name (One through 255 characters, composing the host name and domain name; the domain name is always associated with an IP address.)
The recipient and domain name are separated by an @ sign. Valid characters are letters (a through z), digits (0 through 9), period (.) and the hyphen (-). No distinction is made between lowercase and uppercase characters and, of course, spaces are not allowed.
As an aside, if you did want to read the RFC, you can find it here.
This was first published in October 2002