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Proofpoint — Outbound/Inbound Email Communication Process

Understanding the path a message takes from sender to receiver is what makes the rest of Proofpoint's configuration (URL Defense, Attachment Defense, imposter protection) make sense — each control sits at a specific point along this path.

The lifecycle of an email

  1. The lifecycle starts at the MUA (Mail User Agent) — the client the sender uses to compose and send the message, such as Outlook.
  2. The message goes to the internal MTA (Message Transfer Agent) — the mail transfer agent inside the sender's own network.
  3. The internal MTA transfers the message to the organization's mail server, which is also an MTA.
  4. The sending mail server transmits the message across the internet to the receiving organization's mail server.
  5. The receiving mail server passes the message to the internal MDA (Mail Delivery Agent).
  6. The receiver retrieves the message from their mailbox.
flowchart LR
    A[MUA<br/>e.g. Outlook] --> B[Internal MTA]
    B --> C[Sender's Mail Server<br/>MTA]
    C -->|Internet| D[Receiver's Mail Server]
    D --> E[Internal MDA]
    E --> F[Receiver's Inbox]

Note

The source notes describe this lifecycle to explain the general path a message takes, without specifying at which exact hop Proofpoint's own gateway sits. As a cloud email security gateway, Proofpoint's filtering (URL Defense, Attachment Defense, imposter detection) is applied to mail as it transits the MTA stage — before the message reaches the receiver's inbox.