Like the recipient, the sender can have an agent. This allows the sender to disconnect from the Internet while the agent forwards the email.
There may be multiple reasons the recipient, or his agent, to not be accessible. The sender’s agent can retry sending the email even when the sender is offline.
When Alice or Bob use an agent, from the perspective of Alice, the email flow is identical to the direct method.
The Kozlowski Family ● An Attempt at Restoration ● Family.Kozlowski