The Official Standard RFC 2822

The email format is formally defined in RFC 5322. It describes the syntax for email addresses. The name must be a unique name within the domain and the two components are separated by an @ character. The domain specifies which server the email should be delivered to. The length of the domain name is limited by a maximum of 253 characters and the local-part of an email address may be up to 64 characters long.

DNS validation

DNS Validation attempt to verify that the domain has a MX record. While most domains do have DNS MX record entries some do not. MX is short form for Mail exchanger record. The validation results display the hostnames and IP addresses for the requested domain.

SMTP validation

When the mx server has been resolved, the connection can be established to the remote host. During the SMTP dialogue, various checks and commands can be performed. The first SMTP command is a EHLO command. This is known as the Hello greeting. The client notifies the receiver of the email address of the message in a MAIL FROM command. If the server returns a result code 250, the client can proceed. The QUIT command ends the SMTP session.

Internationalized email

Traditional email addresses are limited to characters from the ASCII character set. Therefore, it is not possible to use international UTF-8 characters. In 2009, ICANN decided to implement a new class of top-level domains, assignable to countries and independent regions, similar to the rules for country code top-level domains. Here are some internationalized email examples:

• mailtest@مثال.إختبار (Arabic)
• mailtest@例子.测试 (Simplified Chinese)
• mailtest@παράδειγμα.δοκιμή (Greek)
• mailtest@пример.испытание (Cyrillic)
• mailtest@בײַשפּיל.טעסט (Hebrew)