PRTG Manual: Monitoring Email Round Trip

Email Round Trip sensors ensure the end-to-end delivery of emails and make it possible to monitor availability and performance of a complete email delivery process. There are two sensor types for this task:

Both initially deliver an email to a mail server using SMTP. Afterwards the receiving mailbox is scanned using Post Office Protocol version 3 (POP3) or Internet Message Access Protocol (IMAP) until the email arrives. The test email contains a unique code in the topic that is used to identify the email, such as PRTG Roundtrip Mail: {6D136420-5A3E-46CF-871A-1DAF0C4F3D5D}

When PRTG successfully receives an email in this email round trip cycle, it marks the respective message for deletion on the mail server. Usually, a mail server will then delete this email. For best performance, we recommend using a dedicated email accounts for email round trip sensors.

Monitoring an Email Round Trip
Monitoring an Email Round Trip

In the scenario shown above, there are three steps in the round trip:

  • Step 1
    PRTG delivers an email via the SMTP protocol to a mail server (just like an email client).
  • Step 2
    The SMTP server delivers the email to a POP3/IMAP server (which can be located at a remote site, in your local LAN, or on the same server as well).
  • Step 3
    Every few seconds PRTG connects to the POP3/IMAP server until the test email arrives.

Recommended Configuration

Here is a simple concept to check the delivery of email out of and into your organization:

  1. Create a dedicated email account for this test in your mail system.
  2. Set up an external email account (hosted mail server, free mail service, etc.) and configure it to send all emails back to this dedicated email account in your organization (that you created in Step 1).
  3. Set up a PRTG round trip sensor to send an email to the external email account (that you created in Step 2) using your network's mail server and then check for the email account on your mail system (that you created in Step 1) for the arrival of the email.

With this technique, you are testing multiple aspects of your email server setup. As long as the sensor shows a green light, this means:

  • Your email server accepts emails via SMTP.
  • Emails are being sent to the outside world (internet connection works, MX lookups work, etc.).
  • Emails from an external mail server can be delivered into your mail system (this includes aspects like the fact that the MX records for your domain are correct, your mail server can be reached from the outside world, your email filtering is working, etc.).
  • Emails can be retrieved using POP3 (or IMAP).

Please use dedicated email accounts with this sensor type. If you use more sensors of this type, please make sure that each sensor uses its own email accounts.


These two sensor types are a great tool to ensure the delivery of email from and to your mail servers. Compared to the standard SMTP, POP3, and IMAP sensors, which only check the availability of these services, the two roundtrip sensor types actually monitor the complete transaction from accepting the email on the first email server to the delivery of the email on the final POP3/IMAP server.