RFC 3696, "Application Techniques for Checking and Transformation of Names", February 2004

Errata ID: 1690
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Dominic Sayers
Date Reported: 2009-02-22
Verifier Name: Nevil Brownlee
Date Verified: 2010-04-03

Section 3 says:

(from erratum 1003)

In addition to restrictions on syntax, there is a length limit on
   email addresses.  That limit is a maximum of 64 characters (octets)
   in the "local part" (before the "@") and a maximum of 255 characters
   (octets) in the domain part (after the "@") for a total length of 320
   characters. However, there is a restriction in RFC 2821 on the length of an
   address in MAIL and RCPT commands of 256 characters.  Since addresses
   that do not fit in those fields are not normally useful, the upper
   limit on address lengths should normally be considered to be 256.

It should say:

In addition to restrictions on syntax, there is a length limit on
   email addresses.  That limit is a maximum of 64 characters (octets)
   in the "local part" (before the "@") and a maximum of 255 characters
   (octets) in the domain part (after the "@") for a total length of 320
   characters. However, there is a restriction in RFC 2821 on the length of an
   address in MAIL and RCPT commands of 254 characters.  Since addresses
   that do not fit in those fields are not normally useful, the upper
   limit on address lengths should normally be considered to be 254.

Notes:

I believe erratum ID 1003 is slightly wrong. RFC 2821 places a 256 character limit on the forward-path. But a path is defined as

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

So the forward-path will contain at least a pair of angle brackets in addition to the Mailbox. This limits the Mailbox (i.e. the email address) to 254 characters.

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