TLDR Answer
Given an gmail address lượt thích...
[email protected]
The length limits are as follows:
- Entire E-Mail Address (aka: "The Path"): i.e., [email protected] --
256characters maximum. - Local-Part: i.e., me --
64character maximum. - Domain: i.e., example.com --
254characters maximum.
Source — TLDR;
The RFC standards are constantly evolving, but if you want a 2009 IETF source in a single line:
...the upper limit on address lengths should normally be considered to tướng be 256. (Source: RFC3696.)
Source — The History
SMTP originally defined what a path was in RFC821, published August 1982, which is an official Internet Standard (most RFC's are only proposals). To quote it...
...a reverse-path, specifies who the mail is from.
...a forward-path, which specifies who the mail is to tướng.
RFC2821, published in April 2001, is the Obsoleted Standard that defined our present maximum values for local-parts, domains, and paths. A new Draft Standard, RFC5321, published in October 2008, keeps the same limits. In between these two dates, RFC3696 was published, on February 2004. It mistakenly cites the maximum gmail address limit as 320-characters, but this document is "Informational" only, and states: "This memo provides information for the Internet community. It does not specify an Internet standard of any kind." So, we can disregard it.
To quote RFC2821, the modern, accepted standard as confirmed in RFC5321...
4.5.3.1.1. Local-part
The maximum total length of a user name or other local-part is 64 characters.
4.5.3.1.2. Domain
The maximum total length of a domain name name or number is 255 characters.
4.5.3.1.3. Path
The maximum total length of a reverse-path or forward-path is 256 characters (including the punctuation and element separators).
You'll notice that I indicate a domain name maximum of 254 and the RFC indicates a domain name maximum of 255. It's a matter of simple arithmetic. A 255-character domain name, plus the "@" sign, is a 256-character path, which is the max path length. An empty or blank name is invalid, though, so sánh the domain name actually has a maximum of 254.