Spam Resource: DELIVTERMS: MTA and MUA

Al Iverson just issued another interesting write-up on Spam Resource:

DELIVTERMS: 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

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