Mail Servers Sensor Types

Using a number of sensors for mail servers you can ensure that your mail systems are working.

Standard Email Sensors

  • SMTP: Monitors availability of SMTP based email servers (Simple Mail Transfer Protocol).
  • POP3: Monitors availability of POP3 based email servers (Post Office Protocol V3).
  • IMAP: Monitors availability of IMAP based email servers (Internet Message Access Protocol).
  • Exchange 2003: Monitors a Microsoft Exchange Server 2003.
  • Exchange 2007: Monitors a Microsoft Exchange Server 2007.

Email Round Trip Sensors

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:

  • SMTP&POP3 Round Trip Sensor
  • SMTP&IMAP Round Trip Sensor

Both initially deliver an email to a mail server using SMTP. Afterwards a mailbox is scanned using POP3 or IMAP until the email arrives. The test email contains a unique code in the topic which is used to identify the email (e.g. "PRTG7 Roundtrip Mail: {6D136420-5A3E-46CF-871A-1DAF0C4F3D5D}").

The graph shows a sample configuration:

  • 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 retrieves emails from the POP3/IMAP server until the test email arrives.


Recommended Configuration

Here is a simple concept to check 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, freemailer etc.) and configure it to send all emails back to this dedicated email account in your organization (which you created in step 1).
  3. Set up PRTG's round trip sensor to send an email to the external email account (which you created in step 2) using your LAN's mail server and then check for the email account on your mail system (which you created in step 1) for 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 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).


These two sensor types are a great tool to ensure 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 mail on the first email server to delivery of the mail on the final POP3/IMAP server.


Keywords: SMTP,POP3,IMAP,round trip,email,mail,mailserver