An email address identifies an email box to which email messages are delivered. An example format of an email address is lewis@example.net which is read as lewis at example dot net. Many earlier email systems used different address formats.

The transmission of email over the Internet normally uses the Simple Mail Transfer Protocol (SMTP), defined in Internet standards RFC 5321 and RFC 5322, and extensions like RFC 6531. Mailboxes themselves are most often accessed using the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP).

The general format of an email address is jsmith@example.org. It consists of two parts: the part before the @ sign is the local-part of the address, often the username of the recipient (jsmith), and the part after the @ sign is a domain name to which the email message will be sent (example.org).

It is not clear from the email address domain name what is the actual destination (the mailbox host) of an email. A mail server will use the Domain Name System, which is a distributed database, to find the IP address of the host of the domain. The server queries the DNS for any mail exchanger records (MX records) to find the IP address of a designated mail transfer agent (MTA) for that address. That way, the organization holding the delegation for a given domain —the mailbox provider— can define which are the target hosts for all email destined to its domain. The mail exchanger does not need to be located in the domain of the destination mail box, it must simply accept mail for the domain. The target hosts are configured with a mechanism to deliver mail to all destination mail boxes.