Why an email address must not exceed 254 characters?

Question

I have a question about the email addresses length. Why in wikipedia or in some other sites tell that the maximum number of chars in an email-user-name is 64, that the server-name must have maximum 255 chars and that together the user-name-mail@server-name must not exceed 254 characters?

If it must have 254 chars, why the server-name must not exceed 255??? I don't understand that...

Could you help me please? Thanks!

Answer 1

The relevant SMTP standard is currently RFC5321. Section 4.5.3 describes the limits. The length of a mail path can't exceed 256 bytes. Since a mail path includes angle brackets, practically this means the user@host portion can't exceed 254 characters.