Sales Loves MarketingA mail user agent (MUA) is the program you use to handle your own email: you write, send, read, search and file messages in it. Most people simply call it an email client. Microsoft Outlook, Mozilla Thunderbird, Apple Mail and the Gmail website are all mail user agents.
How does a mail user agent work?
The Internet Mail Architecture (RFC 5598) describes the MUA as the part of the email system that works on behalf of the user. It has two roles: the author's MUA creates and sends a message, and the recipient's MUA receives and shows it. A single email passes through these steps:
- You write the message in your MUA and press send.
- Your MUA submits it to your provider's submission server, usually over an encrypted connection on port 465 or 587. RFC 8314 recommends encrypted connections for both sending and reading mail.
- Mail transfer agents relay it. Your provider's mail transfer agent (MTA) looks up the recipient domain's MX record in DNS and passes the message on, sometimes through several servers.
- The recipient's provider delivers it into their mailbox.
- The recipient's MUA fetches it using IMAP, POP3, JMAP or the provider's own protocol, and displays it.
MUA vs MTA
| Mail user agent (MUA) | Mail transfer agent (MTA) | |
|---|---|---|
| Who uses it | The person sending or reading email | Mail servers |
| Main job | Write, send, read and organise messages | Relay messages between servers |
| Visible to the user | Yes | Rarely |
| Examples | Outlook, Thunderbird, Apple Mail, Gmail | Server software run by email providers |
Some MUAs can send mail directly, but in normal use the MUA hands the message to a server, and the MTAs do the relaying.
Examples of mail user agents
- Desktop apps: Microsoft Outlook, Mozilla Thunderbird, Apple Mail, Evolution and KMail.
- Webmail and mobile apps: the Gmail and Outlook.com websites and apps, which act as MUAs inside a browser or phone.
- Text-based clients: Mutt and Alpine. Alpine replaced the older Pine client, which is no longer developed.
Why mail user agents matter in email marketing
The MUA decides what your recipient actually sees. Clients support different HTML and MIME features, so the same newsletter can look different in Outlook and in Gmail. Test campaigns in the clients your audience uses most.
MUAs also affect your metrics. Apple Mail's Mail Privacy Protection can hide the reader's IP address and download remote content, such as tracking pixels, in the background when a message arrives rather than when it is viewed. That makes open data less reliable, so clicks, replies and conversions are better measures of engagement.
Related terms
Frequently asked questions
What is the difference between an MUA and an MTA?
The MUA is the program the person uses to write and read email. The mail transfer agent (MTA) is server software that relays the message from one mail server to the next until it reaches the recipient's provider. Users see the MUA; they almost never see the MTA.
Is Gmail a mail user agent?
The Gmail website and apps act as mail user agents, because they are what you use to write and read messages. Behind them, Google runs the servers that submit, relay and store the mail.
Why should email marketers care about mail user agents?
Each MUA renders HTML email differently and handles images and privacy in its own way. That affects how your email looks and how reliable your open rates are. Apple Mail's Mail Privacy Protection, for example, can download images in the background before a person opens a message.