Created on 2005-07-14 by Rainer Gerhards.

ALiveMon's TCP monitor can be configured for application-level monitoring of mail* servers. Mail servers can be monitored with a TCP probe. There are two ways to do this: either via "plain" TCP monitor or via one that is protocol-specific.

With the plain monitor, AliveMon connects to the mail server port and checks if the machine accepts connections. If you know it does, you already have very good impression about the machine's state (much more than with a simple ping monitor). But there are still chances  that the mail server is not fully operational. And besides that, you might experience issues if you have a transparent virus scanning proxy installed (see our FAQ for more details).

So a better way is to create an application specific TCP monitor, the second alternative. With this option, you actually "speak" SMTP, which is the mail server's mail reception protocol. With that option, not only tries AliveMon to connect to the server, it also starts a mail delivery transaction. When the server replies to the start command (which is "HELO"), we know that it actually is acting on application-level commands. This is an extremely good indication that the server is fully alive.

We recommend application-specific settings. Below is a screen-shot with the details. Please note that a mail server will respond with "220 ...something..." to a HELO request (this is defined in the SMTP protocol).

As a side-note, we do not actually try to deliver mail (and check that it can be received). That, of course, would be the ultimate test. In theory, it could happen that the server dies at some point after initiating the mail delivery but before mail is actually transmitted. In my experience, however, I have never seen a server failing at such a later point - with one exception: an errornous anti-spam configuration could can cause a server to deny service to anyone, even though the mail transaction (and its initiation) is otherwise perfectly-well. One can argue if this is something that a monitoring application should check. Given the complexities and ressource-intensiveness of a full send/delivery cycle check, we at Adiscon have decided to not support this option.

*Almost all modern mail servers use the SMTP protocol, which is used with this TCP monitor.