What is an email client? How email clients work

Every time you check your mail, you use an email client, whether it is the Mail app on your phone or Gmail in a browser tab. Each one is a window onto a mailbox that lives somewhere else, and the difference starts to matter the day you want to switch or something breaks.

What is an email client?

An email client is the program you read, write and organise email in. It fetches messages from the server that holds them, lets you search, file and reply, and hands what you write to a server that sends it.

Its technical name is the mail user agent, or MUA. RFC 5598, the IETF's description of Internet mail architecture, says the MUA "works on behalf of" its users and is "their representative within the email service".

A client is not the server. Your mail sits in a message store run by your mail service, and transfer agents you never see move it between services. RFC 5598 also notes that one MUA can be split, with a thin interface on a phone and most of the work on a server. Webmail is that split taken all the way.

Email client vs webmail vs mail service

The big providers offer all three under one name, which is why the terms blur. Gmail is a mail service, a website and a pair of phone apps.

You can change the client without moving your mail, and an installed client can open several services at once.

Webmail needs no install and is always up to date, but it mostly works online and usually opens only its own service's mail. Gmail's "Check mail from other accounts", which fetched other mailboxes over POP, takes no new users after the first quarter of 2026 and stops in January 2027. Google suggests forwarding that mail instead, or adding the account to the Gmail phone app, which uses IMAP.

Types of email clients in 2026

Every platform has one built in: Apple Mail on Macs, iPhones and iPads, the Gmail app on most Android phones, and on Windows the new Outlook, which replaced the Mail app when Microsoft ended its support at the end of 2024. Third-party clients such as Thunderbird, Spark and Superhuman earn their place with what the built-in app lacks, from one inbox for every account to AI.

How does an email client work?

Every client does the same jobs behind the screen.

How a client finds your mail servers

When you add an account, the client asks for your address and password and works out the rest. Thunderbird, K-9 Mail and other clients follow Mail Autoconfig, in production for about 15 years and now an IETF draft, which sets this order:

1. The provider's own settings file, at https://autoconfig.example.com/mail/config-v1.1.xml.
2. The ISPDB, a central database of known providers.
3. The same lookups for whoever receives the domain's mail, found from its MX record.
4. Other methods the client supports, then the settings you type in.

Microsoft's Autodiscover, for Exchange and Microsoft 365 accounts, tries https://autodiscover.example.com/autodiscover/autodiscover.xml and similar addresses, with an _autodiscover._tcp SRV record as the last resort. RFC 6186 defines SRV records that point straight at the servers, such as _imaps._tcp for reading and _submission._tcp for sending.

The autoconfig draft leaves SRV out on purpose: plain DNS answers can be forged, and an SRV record cannot say which username or sign-in method to use. Nextmail can offer these records for a verified domain, and calendar, contacts and some mail apps read them, but Thunderbird and Outlook still need the server names typed in.

Reading mail: IMAP, POP3 or JMAP

• IMAP keeps the mail on the server and handles folders "in a way that is functionally equivalent to local folders", as IMAP4rev2 (RFC 9051) puts it, so every device sees the same folders, read state and flags. What is IMAP? goes deeper.
• POP3 downloads the inbox to one device, often deleting it from the server, so folders and read state stay on that device.
• JMAP is the newer IETF standard: JSON over HTTPS with cheap sync and push, run today by Fastmail and Stalwart. See What is JMAP?

Gmail and Microsoft 365 also have their own APIs, which their own apps use.

Sending mail over SMTP

IMAP and POP3 only read, and RFC 9051 leaves posting mail to a submission protocol. To send, the client signs in to your provider's submission server over SMTP and hands the message over, on port 465 with TLS from the first byte or on 587 with STARTTLS. The provider's servers then find the recipient's servers, so a client never looks up MX records itself. Over IMAP it usually also saves a copy to Sent, because SMTP keeps nothing. What is SMTP? follows the message from there.

The local cache, offline mode and search

Installed clients keep a copy of your mail on the device: some every message, others the headers first and each body when you open it. That copy lets you read on a plane, and changes you make offline wait until you reconnect. IMAP4rev2 is built to resynchronise such a client.

Search uses the local copy where it can, which is fast but covers only what has been downloaded. Webmail usually searches on the server.

Push notifications and threads

New mail appears quickly because the server tells the client. Over IMAP that is the IDLE command, which lets the server send updates in real time instead of being polled, for the one folder the connection has open. Phones cannot keep a connection open for every app, so mobile clients usually get a nudge through the phone's notification service instead.

Conversations are built from headers. Each message has a Message-ID, and a reply names what it answers in In-Reply-To and References. Where those are missing, clients fall back to the subject, which is why unrelated mail with the same subject sometimes shares a thread.

How HTML email is drawn, and why images are blocked

Most mail is HTML, often with a plain-text copy alongside. The client strips scripts and anything else that could run code, then lays out the rest with its own engine. Each client supports a different slice of CSS, which is why a newsletter can look different in Outlook and Apple Mail.

Images are the privacy problem. Many are links the client fetches from the sender's server when you open the message, and Apple's privacy notice lists what that fetch can tell a sender: when and how often you opened it, whether you forwarded it, and your IP address, which gives away your location. A tracking pixel is an image that exists only to report this. Clients respond in three ways:

• Ask first. Many block remote content until you allow it for a message or a sender.
• Use a cache. Gmail and Yahoo load images through their own caches, so the sender sees the cache rather than your device, though the open still counts.
• Fetch everything. Apple's Mail Privacy Protection downloads every message's remote content in the background, through two relays with different operators, opened or not.

Email client security and privacy

• Encryption on the wire. RFC 8314 (2018) told clients and providers to retire cleartext and prefer TLS from the first byte: port 993 for IMAP, 995 for POP3 and 465 for sending. TLS protects the hop to your provider, not the stored message. That takes end-to-end encryption such as OpenPGP or S/MIME.
• OAuth instead of passwords. Signing in the old way sends your username and password with every request, and the app often stores them, which makes them easier to steal. With OAuth you sign in on the provider's own page and the client gets a token. Microsoft switched the old way off for every Outlook.com account on 16 September 2024. Where a provider offers mail apps no OAuth, an app password for one app, revocable on its own, is the fallback.
• Open tracking. Litmus does not count Apple's protected opens as reliable, and puts Mail Privacy Protection behind roughly 55 to 60% of all opens.
• Phishing. SPF, DKIM and DMARC decide whether a From domain checks out, and BIMI lets some clients show a checked brand's logo.

What AI changed in email clients

Summaries of long threads, suggested replies and drafts in your own voice are now ordinary, and the big built-in apps include much of it at no extra cost. The bigger change is assistants that act: they file and label mail, draft and send answers, run instructions on a schedule or when a message arrives, and some let outside agents into the mailbox over MCP. That adds two questions for any client: can it send without asking you, and how much of your mail can it reach?

Our 2026 comparison of AI email clients ranks the apps themselves.

How to choose an email client

If you have one address and like your provider's website, webmail is a perfectly good client. Otherwise, check:

• Your mail. A client that speaks your provider's own protocol, as Outlook does with Microsoft 365, gets features plain IMAP does not carry. Elsewhere, look for solid IMAP, or JMAP where it is offered.
• Your devices. It should run on every computer and phone you use, ideally with your signatures and rules following you.
• Your team. A shared address such as support@ needs assignment, comments the customer never sees, and a sign that a colleague is already replying.
• Privacy. See how it treats remote images and tracking pixels, and where its AI runs.
• Price. Built-in clients are free. Many others charge per person per month, which adds up across a team.

Common questions

Is Gmail an email client?

Gmail is a mail service with its own clients: the website, and apps for Android, iPhone and iPad that can also open other accounts over IMAP. A gmail.com address works in any other client too, and Litmus counts an open by the app it was read in, not by the address.

Is Outlook an email client?

Yes. Outlook is Microsoft's client for Windows, Mac, the web and phones, and Outlook.com is also the name of Microsoft's free mail service. Litmus counts them apart: in September 2026, 3.48% of opens were in desktop Outlook and 0.29% in Outlook.com in a browser.

What is the most used email client?

By opens, Apple. Litmus's figures for September 2026, from over 1.5 billion opens of its customers' emails, put Apple at 76.56%, Gmail at 16.10%, Outlook at 3.48% and Yahoo Mail at 2.35%. Those are opens, not people: Litmus sees a client only when it loads a message's images, and Apple's share includes Mail Privacy Protection, which loads them for every message, read or not.

Where Nextmail fits

Nextmail is a mail service and a client in one. Point a domain at it and read its mail on the web and on iPhone and Android. The assistant works on the same mail: ask your inbox to find, file, label or answer messages, and it asks before it sends, while tasks run on a schedule or when mail arrives. On a paid plan, a team can assign a conversation on a shared address, discuss it in comments the sender never sees, and mark it done.

It also works with other clients. Every address that keeps its mail in Nextmail opens in Apple Mail, Outlook, Thunderbird or the Gmail app over IMAP, SMTP or POP3 with TLS, signed in with an app password for that one address. If your mail stays at Gmail, Outlook, Yahoo or iCloud, connect that mailbox and work it from Nextmail instead. Or build your own client on Nextmail.