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How can I monitor email round trip when I can't use PRTG to check the destination mailbox



I have a PRTG installation on the Internet. I want to check that mailflow is working correctly into and out of my internal exchange system. However, I don't expose any pop3 or imap mailboxes to the internet, so my PRTG installation can't check for email arrival using the standard SMTP/POP3 round trip check.

I would like to:

  • a) have PRTG validate my SMTP gateway is up by sending it an smtp message.
  • b) have that SMTP message relayed to my internal mailserver
  • c) have the internal mailserver redirect the email to a pop3 mailbox located on the internet
  • d) use PRTG to check for the arrival of that return message and alert if it either doesn't arrive within a specified time, or takes too long to arrive.

Ideas anyone?

email imap mail-flow pop3 prtg round-trip smtp

Created on Apr 7, 2010 11:43:36 PM

Last change on Apr 8, 2010 7:25:45 AM by  Daniel Zobel [Product Manager]

2 Replies

Accepted Answer



Dear Charlie,

with the standard SMTP&POP3 Round Trip sensor, you can define SMTP and POP3 servers independently from each other.

In your description, only at step a) and d) PRTG is involved. In step a), PRTG can send an email to your internal SMTP server. Of course, PRTG will need to check the external mailbox at step d), which should not be a problem, as it is located on the Internet, right?

So, in the PRTG web interface,

  • Create a new device with the DNS or IP address of your internal SMTP server.
  • On this device, create a new SMTP&POP3 Round Trip sensor - the "Add Sensor" screen will appear.
  • Under section "Step 1...", enter necessary credentials for your internal SMTP server.
  • Under section "Step 2...", enter DNS or IP address of the external POP3 server on the Internet and enter necessary credentials.

Alternative option for installations with no direct Internet access

If you cannot access the Internet from your PRTG installation (e.g. due to company policy), please consider altering step d) and adding an additional step e), as the following:

  • d) Have the external POP3 mailbox forward the email to another internal POP3 account immediately (but make sure that the email's subject is not changed by the forwarder!)
  • e) Use PRTG to check for the arrival of that forwarded message in your internal POP3 mailbox and alert if it either doesn't arrive within a specified time, or takes too long to arrive.

When using this alternative option, you'll have to enter the internal (not external) POP3 server's details in "Step 2..." (see above).

Created on Apr 8, 2010 7:37:49 AM by  Daniel Zobel [Product Manager]

Last change on Apr 8, 2010 12:05:03 PM by  Daniel Zobel [Product Manager]



Thanks, this is a great method to monitor Exchange servers without having to consume another mailbox license and without having to open imap/pop3.

Created on Jan 3, 2022 11:13:43 PM

Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.