According to RFC 2822 (Section 3.2.4) - [http://www.ietf.org/rfc/rfc2822.txt], The local part of an e-mail address may consist of alphabetic and numeric characters, and the following characters: !, #, $, %, &, ', *, +, -, /, =, ?, ^, _, `, {, |, } and ~, possibly with dot separators (.), inside, but not at the start, end or next to another dot separator: {quote}dot-atom-text = 1atext *("." 1atext){quote}

The expression used in Zend_Validate_EmailAddress misses the "next to another dot separator" part. For example, the following e-mail address validates:

Abc..123@example.com

Further, according to RFC 2821 (Section 4.5.3.1) - [http://www.ietf.org/rfc/rfc2821.txt], {quote}The maximum total length of a user name or other local-part is 64 characters.{quote}

An email address with 65 characters in the local part validates.