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Email System Terminology
Table 1 shows some acronyms commonly used in discussions of email systems. Four of these (MUA, MSA, MTA, and MDA) may be a bit confusing. The confusion arises over the word agent, which is used to mean a process, a program, or a machine, rather than an individual or organization. The use of plain English words like actor and agent to mean something else is common in computer science. This distortion of meaning occurs naturally, as specialists try to be concise in their communications. Instead of saying "MSA process", meaning the process that performs the functions of an agent that handles mail submission, it was easier to just use MSA as an acronym. It works as long as the context is clear.

In these articles, we talk about the whole spectrum of "agents", from organizations and individuals to programs and processes, so we can't just use the common jargon without clarification. We also need to make distinctions based on function, like transmitter vs receiver, instead of just calling every relay an MTA.

Figure 2 is the same as Figure 1, but with the processes labeled using the acronyms in Table 1.