The contents of a bracketed domain can have a \ precede a character to escape it, and the following character must not be 10 (LF) or 13 (CR).Bracketed domains must start with, and must not contain characters 9 (TAB), 10 (LF), 13 (CR), 32 (space), 91-94 (, ^).A label must not start nor end with a hyphen. A label may contain hyphens, but no two hyphens in a row.The maximum length of a label is 63 characters.No two periods in succession can be in a domain name. No period can start or end a domain name. A plain domain consists of labels separated with periods.If an email is using the obsolete quoting on a per-label basis, then the email address consists of unquoted or quoted chunks separated by periods.This supersedes the previous rule, allowing spaces and quotation marks in the email address as long as they are escaped.
If the quoted local part has a backslash, the following character is escaped and must not be 10 (LF), 13 (CR).The contents of a quoted local part can not contain characters 9 (TAB), 10 (LF), 13 (CR), 32 (space), 34 (“), 91-94 (, ^).The quoted local part starts with a quotation mark, ends with a quotation mark.TEXT can contain alphabetic, numeric, and these symbols: !#$%’*+-/=?^_`~ ( RFC 2822, section 3.4.1).No periods can start or end the local part. Unquoted local parts can consist of TEXT, optionally separated by periods.The local part can be unquoted, quoted in its entirety, or quoted on a per-label basis.Email addresses consist of a local part, the symbol, and the domain.All email addresses are in 7-bit US ASCII.The summary is based largely on RFC 2822. Here’s a list of the principal RFC texts about email addresses and the SMTP standard:
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