I like the uid idea or even having a unique passphrase in the subject of the email.
Granted, I'm thinking of monitoring systems on a much larger scale where even a single instance of spamming can keep the entire dev team up at night. I do not see this being a problem for the smaller targeted audience that you are going after that will initially only set up a handful of alarms.
The email addresses for alarms are editable; if you get spam to one of these addresses, you can change the address. We were also thinking of adding an option to obscure the email address by adding a uid to the end. An example would be [email protected].
We also offer regex-based filters on both the subject and the body, so you can configure an alarm to only trigger if a certain keyword appears in the message.