The structure of the SMTP protocol allows for multiple recipient agents.

The agents can be in the same room or in other countries.

The sender will attempt to connect with one, and if that fails try the next.

This technique allows for reliable messaging even when an agent fails or is inaccessible.

For Bob to get his emails, many options are possible.

Bob’s computer can individually check all agents.

The agents could share the emails they receive with each other so that polling any gets them all.

An additional layer is added where the agent Bob communicates with does the polling to the other agents.