Call for Deliverability Monitoring Vendors: Postmastery

Hey, deliverability monitoring vendors! I know you're out there, and I don't know a lot about many of you. Do you manage a service that does inbox deliverability testing, seedlist testing, inbox monitoring, or similar functionality? Feel free to drop me a line and let me know a little bit about your product or service, and I'll publish it here. 

First up -- Willem Stam has kindly provided the following information about Postmastery: