In this illustration, user agents are represented by desktop computers and mobile phones, while mail servers are represented by servers. Mail servers are further depicted as having a long rectangular slot containing a series of white envelopes-representing the outgoing message queue-and three smaller slots containing two white envelopes-representing the user mailbox.

The image depicts three groupings of user agents and mail servers, arranged in a triangular formation. The first and second mail servers are connected by a blue arrow that points back and forth between them. The second and third mail servers are similarly linked, as are the third and the first. These arrows are labeled "SMTP."