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Email address syntax

Last Edit: 04/04/17

The syntax of an email address can be broken into three sections:

The three sections are as follows:

  1. joe
  2. @

The 'local' segment of the email address is joe, and is the recipients username at any given mail service. The username at each mail service is unique: for example, only one user can have the username joe at However, the username joe can be registered at other mail services (domain names). One problem with webmail services is that the 'best' usernames are already taken, and is has meant that users have to either purchase a personalised email address or use mail services ( that have multiple domain names that users can register their preferred username on.

The '@' character is a abbreviated symbol for 'at', such as ' joe at'. It connects the user to the machine. The '@' sign is also the one character, of any email address, that remains the same. Ray Tomlinson was responsible for proposing the use of the '@' sign in 1971.

Part 3 of an email address is the hostname: The hostname for modern email addresses is a domain name - historical email addresss did not use domain names because the DNS did not exist in the 1970's - and is used by a Mail Transfer Agent (MTA) is locate the correct mail server to send an email message to. Domain names typically contain two sections: the second level domain (example) and the top level domain (net). Email addresses can contain third, fourth and fifth (up to 27) level domains: such as which includes a third level domain (co). Most email users have an email address registered with a global service, such as:,,,,, and These global services are usually free - supported by ad revenue - and it means the user does not have to rent a mail server but has to use the domain name of the service. However, email users can register a second level domain - through ICANN accredited domain name registrars - and rent a mail server to provide mail services for the domain name. This means that the user has a unique and personalised email address - useful for business purposes - but has the cost of the registering the domain name and renting mail services.