Jean-Christophe Boggio writes:

This is definitely wrong.

The syntax "@example.com: localuser" rewrites all addresses of the form "us...@example.com" to "localuser-user@localhost", where "localhost" is your local mail domain.

Therefore, with your example, <us...@external.com> gets rewritten as <some...@somewhere.com-user@localdomain>, and Courier begins looking for a local account called "some...@somewhere.com".

The rest of your E-mail is hard to follow. Rather then provide actual data, you chose to replace vital information with randomly-generated substitute text, which does not appear to be even consistent from paragraph to paragraph. As such it's really impossible to analyze what your configuration files really contain.

Try again, this time giving the actual contents of your configuration files, instead of made-up names. What you posted was mostly unreadable, except for the single obviously wrong portion, mentioned above.