Monitoring services for email servers

Does anyone have experience of hosted services for monitoring the status of email servers?

I host my own email on a Linux VPS. If either my SMTP or IMAP servers stop responding to requests, I want to receive an SMS within 10 minutes telling me so.

Better still, I'd like to be able to set up arbitrary TCP banner checks (i.e. periodically connect to a specified port and verify that it receives a specified string). I know that software exists that does this - monit, nagios, etc - but I don't want to host or maintain it myself.

Subscription services are fine (indeed I'd expect to pay something for this).

Update: I previously stated that Pingdom only supports HTTP, but it does actually support SMTP and IMAP as well. Thanks @fmu on Twitter for pointing this out!

From serverfault Sam Stokes

• DynDNS NetMon is $9.99/mo or $99.99/yr and supports {SMTP,IMAP}{,S}. It doesn't seem to support arbitrary TCP banner checks.

  It supports notification to "e-mail or pager aliases"; I'm not sure what a "pager alias" is, but it doesn't appear to support SMS notification. It could be combined with something like Clickatell to provide SMS support.

  (This is answering my own question, but I'd like to see if anyone knows any other services that do this. Also, if you've actually used this service, please edit or comment on this answer.)

  From Sam Stokes

• You can monitor the services using SNMP solarwinds has a product for this, However if you want to check the actual mail flow, which would be far reaching than just monitoring ports you could write a vbscript to do a mail flow check, put it on a loop say every 5 mins and report any errors.

  From Nasa

• Pingdom has several price plans including a free plan, supports multiple check types and SMS notification.

  From Sam Stokes

• I've tried a few of these. Pingdom has some nice features and isnt so pricey but it has it's issues. The way it monitors email does not catch every fault. It may tell you your email is functioning but people are receiving bounce back messages or your emails are not going through. Im researching a few other sites on their effectiveness and will let you know my findings

  From