SMTP monitoring

Is there a SAM monitor for monitoring SMTP emails round trip or at least to see if an email got rejected for a non microsoft email server?

I notice there is a POP3 email checker and i have downloaded the Exchange round trip email checker but we have an SMTP server that is running on Linux and we would like to check that it is processing SMTP emails or at least that it is not rejecting the emails, but there doesn't appear to be a monitor for it.

Thanks