What Is an Email Client?

In short, an email client is a computer program used to read and send electronic messages. An email client isn't the same as an email server, however; the latter is the hardware that transports and stores mail centrally for an email provider's many users. An email client, by contrast, is what a single user like you interacts with. Typically, the client downloads messages from the server for local use (or for use within a browser) and uploads messages to the server for delivery to its recipients.

The email client lets you read, organize, and reply to messages as well as send new emails.

To organize email, email clients typically offer folders, labels, or both. An integrated search engine lets you find messages by details such as senders, subjects, times of receipt, and content.

In addition to email text, email clients also handle attachments, so you can send and receive computer files (such as images, documents or spreadsheets) via email.

Email clients can use a number of protocols to send and receive emails via email servers.

The messages are either stored locally on your computer (typically when POP, or Post Office Protocol) is used to download mail from the server), or emails and folders are synchronized with the server (usually when the IMAP and Exchange protocols are employed). With IMAP (Internet Message Access Protocol) and Exchange, email clients accessing the same account see the same messages and folders, and all actions automatically synchronize.

To send an email, email clients use SMTP (Simple Mail Transfer Protocol) almost exclusively. (With IMAP accounts, the sent message is usually copied to the "Sent" folder, and mail clients can access it.)

Other email protocols exist, too. Some email services offer APIs (application programming interfaces) for email clients to access mail on their servers. These protocols may offer additional features such as delayed sending or the temporary setting aside of emails.

Historically, X.400 was an important alternative email protocol in use primarily during the 1990s. It's sophistication made it suitable for governmental and business use but harder to implement than SMTP/POP email.

With web-based applications that access email on a server, browsers turn into email clients.

If you access Gmail in Mozilla Firefox, for instance, the Gmail page in Mozilla Firefox acts as your email client; it lets you read, send, and organize messages. The protocol used to access the email, in this case, is HTTP.

In one technical sense, any software program that accesses email on a server using POP, IMAP, or a similar protocol is an email client. So, any software that automatically handles incoming email can be called an email client (even when nobody ever gets to see the messages), especially in relation to the email server.

Popular email clients include Microsoft Outlook, Mozilla Thunderbird, macOS Mail, IncrediMail, Mailbox and iOS Mail. The most popular web-based email client is Gmail; others include Yahoo! Mail and Outlook.com.

Historically important email clients have included Eudora, Pine, Lotus (and IBM) Notes, nmh, and Outlook Express.