The best approach probably is to make sure SMTP is available on e.g. port 25. The SMTP sensor (default PRTG) might be more sufficient for this.
Another good idea would be to monitor the directories on the IIS itself.
A word of advice out of personal experience with the IIS SMTP engine - it is not actually very powerful nor robust - it has its quirks. If you want something cheap, actually free, look at this alternative service for SMTP that gives you a lot more options, works more stable and is easier to administrate as well:
It is a pretty famous SMTP open source project that really does a good job - have used it a lot for internal SMTP relay servers with great success and eliminating a lot of the issues and challenges with IIS. I monitor the h-mail service, the SMTP port/connection and the SMTP queue directory for to many files and to old files (e.g. 1 hour and above) to see if anything got stuck and or stressing the system.