I don't know if anyone else has tripped over this already, in 0.37.2 (I know, I know, it's old):

In the courier(8) man page, in the section describing the "me" configuration file, there is the following text:

Warning: If you change the contents of this configura- tion file, you must run the makealiases command again, else your mail will promptly begin to bounce. If you don't have this configuration file defined, and you change the system's network host name, you also must run makealiases.

I've discovered that this also applies to the "defaultdomain" configuration file, too, for mail sent to aliases of system accounts.

William