What Is a Domain in an Email Address?

By Roger Golden

An email address consists of four main parts: The username, the "@" symbol, the email domain and the root domain. This tells the mail server who the mail is for, where the message is delivered to and the type of connection being used. The domain section of the address is typically the company you work for, your ISP or a third party service such as hotmail or gmail.


Your username is how you are identified by your email domain. All data packets that are delivered to the network are sorted according to network usernames or IP, short for Internet Protocol, addresses and distributed to the correct location. If the mail server of a network is not able to deliver to a username, the mail is rejected and returns to the sender saying that the user cannot be found.

User Domain

The user domain usually identifies the primary mail server. If you are using Yahoo mail services, then the domain will be mail.yahoo. If you are using a university account, the email address will reflect the institution, and if you are using a company account your email address may be delivered through the company domain. The user domain is similar to a ZIP code on an envelope. Mail can be sent to your general area, or domain, and then routed to individual recipients.

Root Domain

The root domain is represented by the final characters in the email address. Root domains can indicate a commercial root domain with the "com" domain, or represent educational facilities using an "edu" domain. Some countries use a two-letter country code as the root domain, and there are even recommendations for reserving domains for specific types of content. In your email address, the root domain indicates the type of server your email is being relayed through, but has very little bearing on your email.

Using "Mailto:"

Internet email is intrinsic to most platforms. If you type a mailto into the address bar of your browser it will load the system default email program. For instance, you can type "mailto:myaddy@thisdomain.net" to initiate a new email message to someone named "myaddy" at the "thisdomain.net" domain name server. If that email address exists, the mail will be delivered normally. The mailto protocol is one of the original network protocols, along with hypertext transfer protocol, or http, and file transfer protocol, called ftp, among others.