Invalid Email

The IETF set the syntax for email addresses in three Requests for Comment (RFC): RFC 3696, RFC 5321, and RFC 5322.

These RFCs consider an email address invalid when it fails to meet any of the following conditions:

• RFC 3696, section 3 outlines email address syntax restrictions.
• RFC 5321, section 4.5.3.1 defines the parts and lengths of an email address.
  Email addresses consist of two parts: the local-part that comes before the @ sign and the domain part that comes after.
  • The local-part can't exceed 64 octets in length.
  • The domain part can't exceed 255 octets in length.
  • Taken together, the whole email address can't exceed 254 octets in length.
• RFC 5322, section 3 defines the ASCII characters (as octets) allowed in an email address.

These RFCs define length in octets rather than characters.

• A single character in a non-Latin alphabet may use more than one octet, or eight bits, of data.
  For example: the local-part of an email address using Chinese characters can't exceed 32 characters.
• RFC 6531 covers Unicode characters in the local-part of an email address.
  This requires the SMTPUTF8 extension for your email provider.
• RFC 5890 covers Unicode characters in the domain part of an email address.
• Support for international characters depends on the email provider.

• The email address doesn't include the @ symbol between the local-part and domain part.
• The local-part of an email address includes characters other than the following:
  • Letters (A-Z, a-z).
  • Digits (0-9).
  • Special characters (!, #, $, %, &, ', *, +, -, /, =, ?, ^, _, `, {, |, }, ~, .).
• The local-part starts with a period (.) or includes two or more periods in a sequence (first..last@example.com).