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Maximum length of an email address is 254 characters, not 320

I work in a business of constant learning. Technology is continually evolving and being revised. Somethings are rather constant but, being human, weI tend to develop habits occasionally rooted in wrong assumptions. Perhaps a deadline forced a decision without having time to look up the specification and over time, our mind took that unresearched decision as fact. I’ve made numerous databases over the years based upon just such a wrong assumption. My error has been in the acceptable maximum length of an email address. My number is irrelevant but I am in good company with being incorrect on the length of an email address as many people mistakenly believe it to be 320 characters.

An RFC is a request for comments which "is a memorandum published by the Internet Engineering Task Force (IETF) describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems." (Wikipedia) RFCs set the standards that define how the Internet works. RFC3696 and RFC5321 explain that the maximum length of an email address is 254 characters.

There appears to be some confusion over the maximum valid email address size. Most people believe it to be 320 characters (64 characters for the username + 255 characters for the domain + 1 character for the @ symbol). Other sources suggest 129 (64 + 1 + 64) or 384 (128+1+255, assuming the username doubles in length in the future).

This confusion means you should heed the ‘robustness principle’ ("developers should carefully write software that adheres closely to extant RFCs but accept and parse input from peers that might not be consistent with those RFCs." – Wikipedia) when writing software that deals with email addresses. Furthermore, some software may be crippled by naive assumptions, e.g. thinking that 50 characters is adequate (examples). Your 200 character email address may be technically valid but that will not help you if most websites or applications reject it.

The actual maximum email length is currently 254 characters:

"The original version of RFC 3696 did indeed say 320 was the maximum length, but John Klensin (ICANN) subsequently accepted this was wrong."

"This arises from the simple arithmetic of maximum length of a domain (255 characters) + maximum length of a mailbox (64 characters) + the @ symbol = 320 characters. Wrong. This canard is actually documented in the original version of RFC3696. It was corrected in the errata. There’s actually a restriction from RFC5321 on the path element of an SMTP transaction of 256 characters. But this includes angled brackets around the email address, so the maximum length of an email address is 254 characters." – Dominic Sayers

[Source, EPH, Email Address Length FAQ]

Every day I learn something new! (even when I should have known it for a couple of decades)