What is an email client? Apps, webmail and how they work

Client and provider

Your email has two halves. The provider (Gmail, Outlook.com, iCloud, Fastmail and so on) runs the servers that receive, store and send your mail. The email client is the app you use to work with it: the inbox you scroll, the window you write in, the search box you type into.

In technical writing, a client is sometimes called an MUA (mail user agent), and the servers that move mail are MTAs (mail transfer agents). Apple Mail, Microsoft Outlook and Thunderbird are clients. So is the Gmail website, which is a client that runs in your browser.

How a client talks to your provider

Most clients use open standards. They read mail over IMAP (or the older POP3) and send it over SMTP. These work with almost any provider, which is why one app can hold accounts from several companies at once.

Some providers also offer their own interfaces. Gmail has the Gmail API, Microsoft has Microsoft Graph and Exchange protocols, and Fastmail promotes JMAP, a newer open standard (RFC 8620 and RFC 8621). Clients use these for features IMAP does not cover well, such as calendars or Gmail's labels.

Sign-in varies too. Some accounts use your password or an app password, a separate password just for one app. Others use OAuth, where you sign in on the provider's own page and the client receives a token instead of your password.

Webmail vs a desktop or mobile app

Webmail needs nothing installed and looks the same on any computer. A native app runs as its own program, can keep mail on your device for offline reading and fast search, shows notifications, and can put several accounts in one window.

There is a less obvious difference too: where your mail goes. Some third-party apps route your mail through their own servers to add features like scheduled sending or push. That means a copy of your mail, or your sign-in credentials, sits with another company. Other apps connect straight from your device to your provider and keep everything local.

What to look for

Check that the client supports every provider you use. Check where it stores your password (on a Mac, the system keychain is the safe place). Look at privacy defaults, especially whether it loads remote images automatically, since that is how tracking pixels report when you open a message. Look at what happens to your mail: does it stay between your device and your provider, or pass through the app maker's servers? And check the price model, since some clients are subscriptions and others are a one-time purchase.