Networking Protocol Questions Regarding Agent and Mail Server

Question

What protocols are used when

(i) the sender’s agent communicates with the sender’s mail

server

(ii) the sender’s mail server communicates with the receiver’s mail server, and

(iii) the receiver’s mail server communicates with the receiver’s agent?

Answer 1

Thanks for asking your question in such a comprehensive manner, Allen! To answer your question in the general sense: several protocols come into play during different stages of the process, though the heavy-hitter is the Simple Mail Transfer Protocol (SMTP). I will detail the transfer process, per the steps you've outlined, below:

(i) When the sender's agent communicates with the sender's mail server, SMTP is the protocol that's used. SMTP is responsible for transmitting outgoing emails from the sender to the sender's mail server.

(ii) When the sender's mail server communicates with the receiver's mail server, the communication is typically handled, again, by SMTP. However, sometimes Extended Simple Mail Transfer Protocol (ESMTP) will be used, as it offers additional functionality.

(iii) When the receiver's mail server communicates with the receiver's email client or agent, either the Internet Message Access Protocol (IMAP) or Post Office Protocol (POP) will be used, depending on the configuration of said client or agent.

I hope this answers your question(s) sufficiently.