What is the Postmaster address used for?
When you create a new policy within your account, you are required to enter a postmaster email address. This address is used as the sender address when notifications are issued under that policy. Other than that, it is not used. If possible, you should use an address that can be replied to, so that if an end user replies to a notification, the message is delivered.
Note that RFC 2821, which defines SMTP, requires a postmaster address to be maintained for each domain receiving email. This does not have to be the address you provide in the portal.