So bob has sent his message to fairynet, and fairynet has managed to get the message to dragonnet. But how does penny get that message?
Well the last MTA that accepts mail for dragonnet will hand that message to a Mail Delivery Agent (MDA), it is this program that is responsible for storing the messages in a mailbox for penny and allowing penny’s MUA to collect those messages. Up until know the process has been one where the message get’s pushed to the next hop, but once the MDA has placed the message in penny’s mailbox the message must be pulled out for penny to read it.
There are two main ways for a message to get from the mailbox to the user’s MUA. The MDA is responsible for handling whichever the user uses.
One is Post Office Protocol version 3 (POP3). Using this the MUA accepts the messages and stores them locally (earlier versions of Post Office Protocol exist, but are not in wide use anymore).
The other is Internet Message Access Protocol (IMAP). Using this the MUA reads the messages, but they are stored in the mailbox controlled by the MDA.
In both cases the MUA connects to the MDA and pulls the messages when it is ready for them rather than the MDA pushing the messages out.
If penny chooses to respond the whole process starts again, penny’s MUA submits the response to dragonnet’s MSA, a series of MTAs get the message to fairynet and fairynet’s MDA puts the message in bob’s mailbox for bob’s MUA to retrieve.