While a direct connection between Alice and Bob logically works, there are limitations that need to be addressed.

Bob’s computer may not be accessible when the email is sent. Therefore a reliable intermediary or agent is delegated.

From the perspective of Alice, the email flow is identical to the direct method.

However the initial recipient is not Bob’s computer, but his agent’s computer.

Once the email has arrived at Bob’s agent, Bob still does not have the email.

Bob’s computer must be informed or check to see if an email has arrived.

Common protocols for Bob’s computer to check his agent are POP3 and IMAP.

Bob then copies the email from the agent to his local client.

Bob can then delete the email from the agent or leave it there.