Steven D'Aprano wrote:

Email being sent locally may contain zero @ signs, and email being sent externally can contain one or more @ signs. Andy's code:

user, hostname= address.split('@', 1, True)

will fail on syntactically valid email addresses like this:

fred(away @ the pub)@example.com

From Wikipedia:
RFC invalid e-mail addresses
   * Abc.example.com (character @ is missing)
   * Abc.@example.com (character dot(.) is last in local part)
   * Abc..123@example.com (character dot(.) is double)
   * A@b@c@example.com (only one @ is allowed outside quotations marks)
   * ()[]\;:,<>@example.com (none of the characters before the @ in this example, are allowed outside quotation marks)

Your example is valid email address if and only if it is enclosed in quotation mark: "fred(away @ the pub)"@example.com