From Mercury help, Core module configuration:

Username of postmaster  Every system capable of receiving Internet mail must have a user called postmaster, to whom problem and status reports are sent. The postmaster account is usually an alias to a real user on your system, and this is the expectation within Mercury. Enter in this field the username of the user on the machine where Mercury is running who is to act as your postmaster. While it is permissible to have a non-local address as your postmaster address, we strongly recommend you do not do this, since it can create real problems and mail loops when the remote machine is unreachable. This setting is mandatory - Mercury cannot run properly without it.

The nice little app for remote management of Mercury could use some enhancements to the documentation. For example, it says that to access the management functions of HTTPServer one should log into the "Postmaster account". Well I logged into my account which has admin privilages but that didn't help. What is the Postmaster account?