Custom monitor for SMTP servers

I have dug through some of the forums and have yet to find the solution I'm looking for. I have a set of load balanced SMTP servers in my LTM with version 9.3.1 software installed. I have the SMTP health check enabled, however, it doesn't do exactly what I would like it to do. For example, I would like the F5, as part of the health check, to send an actual e-mail to the SMTP server and monitor the response to determine if the e-mail was successful or failed. I recently had an issue where /var on one of the systems filled up, and while the server was still responding to SMTP requests as one would expect, it was failing to accept any e-mails due to the drive being full. I would like the check to disable a node if it fails to receive an e-mail. If you could point me in the right direction, it would be greatly appreciated. Thank you in advance.