Courier Mail Server has many interesting features. One of the features is support for backscatter blacklist. It helps to protect the server from receiving emails which are not possible to deliver (e.g. in case of some problems with recipient’s account).

The common error message in that case is:

Courier: 556 Address unavailable

It’s not very straightforward how should an administrator resolve error 556. Even after fix of user’s account configuration, the message might appear and the Courier won’t accept an email even after restart. It could be even more confusing when Courier accepts email from some subnets, but it returns 556 Address unavailable on other subnets.

The resolution of the problem is simple: fix backscatter database.

Display list of addresses with on backscatter blacklist:

courier show all

Remove problematic address from the list:

courier clear user@domain

You can remove all addresses:

courier clear all

You can find more about backscatter blacklist in Courier’s documentation.