What is an MUA (Mail User Agent)?
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A Mail User Agent (MUA) is the technical term for what most people call an email app or email client. It's the program you use to write, read, send, and organize your email. If you've ever opened Gmail, Outlook, Apple Mail, or Thunderbird, you've used an MUA.
The MUA is your interface to email. It doesn't deliver messages itself (that's the MTA's job), but it talks to mail servers to send and fetch your messages. When you hit send, your MUA hands the message to a mail server. When you check your inbox, your MUA fetches messages from the server using protocols like IMAP or POP3.
MUAs come in two main flavors: desktop apps (Outlook, Apple Mail, Thunderbird) and webmail (Gmail, Yahoo Mail, Outlook.com). Desktop apps download email to your device. Webmail runs in a browser and keeps everything on the server. Both are MUAs. The difference is where your email lives and how you access it.
Why senders care about MUAs: different email clients render HTML differently, handle images differently, and display your sender name differently. An email that looks perfect in Gmail might break in Outlook 2016. Understanding that your recipients use different MUAs helps you test across platforms and avoid rendering disasters.
The MUA is also where spam filtering happens for many recipients. Gmail's spam filter runs inside the Gmail MUA. Outlook's junk folder is managed by the Outlook MUA. So when you think about deliverability, you're thinking about how different MUAs treat your messages.
If you're curious about how your message moves from your MUA to your recipient's MUA, check out what happens when you press send.
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