What is the maximum length of a valid email address?

An email address must not exceed 254 characters.

This was accepted by the IETF following submitted erratum. A full diagnosis of any given address is available online. The original version of RFC 3696 described 320 as the maximum length, but John Klensin subsequently accepted an incorrect value, since a Path is defined as

Path = "<" [ A-d-l ":" ] Mailbox ">"

So the Mailbox element (i.e., the email address) has angle brackets around it to form a Path, which a maximum length of 254 characters to restrict the Path length to 256 characters or fewer.

The maximum length specified in RFC 5321 states:

The maximum total length of a reverse-path or forward-path is 256 characters.

RFC 3696 was corrected here.

People should be aware of the errata against RFC 3696 in particular. Three of the canonical examples are in fact invalid addresses.

I've collated a couple hundred test addresses, which you can find at http://www.dominicsayers.com/isemail

320

And the segments look like this

{64}@{255}

64 + 1 + 255 = 320

You should also read this if you are validating emails: I Knew How To Validate An Email Address Until I Read The RFC

TLDR Answer

Given an email address like...

me@example.com

The length limits are as follows:

• Entire Email Address (aka: "The Path"): i.e., me@example.com -- 256 characters maximum.
• Local-Part: i.e., me -- 64 character maximum.
• Domain: i.e., example.com -- 254 characters 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 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.

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 email 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 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 maximum of 254 and the RFC indicates a domain maximum of 255. It's a matter of simple arithmetic. A 255-character domain, plus the "@" sign, is a 256-character path, which is the max path length. An empty or blank name is invalid, though, so the domain actually has a maximum of 254.

Sadly, all the other answers are wrong. Most of them cite RFC 2821 or newer, which does not even define e-mail addresses. What it does do is define paths. E-mail addresses are defined by RFC 2822 (or newer) and can be much longer. Examples of valid addresses that are not valid paths are:

(Firstname Lastname) user@domain
Firstname Lastname <user@domain>

Both of these are the same mailbox written differently. So if your goal is to store e-mail addresses in a database, a limit of 254, 256 or 320 octets might be too low, although in practise, this is rarely going to be a problem.