The (more or less) regular series here on Spam Resource that defines deliverability terminology. Unlike some other series out there in the world, this one has neither hot questions nor even hotter wings. But I do hope to help shore up gaps in your email and deliverability-related knowledge. Today, I'm going to talk about MTAs and MUAs, mail transfer agents, and mail user agents.

MTA means "Mail Transfer Agent." The acronym MTA is basically interchangeable with the term "mail server." An MTA or mail server is an software application running as server software to handle processing and transmission/receipt of email messages. It's an MTA that responds to a connection on port 25 and answers with “220” and an SMTP banner and accepts or sends inbound or outbound email, transmitted via SMTP. When I look up the MX record for a domain, and I connect to port 25 of that MX record, it is the MTA that answers and responds to my connection attempt.

Examples of MTA software includes Postfix, Sendmail, Exim, PowerMTA and Microsoft Exchange. If you build an email campaign and send it out from your ESP or CRM platform, that platform's email builder application creates the email messages based on your guidance, and hands them off to their own MTAs (mail servers) to handle outbound email delivery.

If you don't run your own email service, you might never interact with MTAs directly. If you're a Gmail user and you use Gmail to email your friend at their Hotmail account, Gmail takes your message, passes it to Gmail's outbound MTA, which then hands it off to Hotmail's inbound MTA, which then accepts the message and puts it in the mailbox used by your friend. You don't actually get to watch this as it happens, but if your friend were to select "view source," "show full headers," or "show original" on the message you sent them, the full headers you reveal would include hidden "received" headers that help trace an email message's path from MTA to MTA.

When somebody asks you for MTA logs, they're typically asking you for SMTP transaction logs -- meaning, show me the output from logging monitoring the email message processing, sending and receiving that a given mail server (MTA) is engaging in. MTA logs for larger messaging platforms can get very big, very quickly, so most folks do not keep log data around for a very long time.

And finally, we can't forget about MUA. MUA stands for "Mail User Agent," meaning an email application (or "client application") typically used by an end user to send, receive and read email. The Mail app on your iPhone is an MUA. Other MUAs include Eudora, Thunderbird, Microsoft Outlook (application), the Gmail application on a mobile device, and if you're old, Eudora or pine. The lines can blur lately between application and service -- if you think about it, Gmail's web user interface is effectively an MUA as a service, as it is both an application and a website.

And of course, don't forget to check out the DELIVTERMS section here on Spam Resource, where we define the common terms used in email technology and deliverability.

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