I just had that problem and spent an hour or 2 figuring out what to do about it. After reading pretty much all of the forum threads on the RS forum and on cpanel forums, I didn´t get any further.

So started digging etc.

Eventually I found out that the customer that was getting these errors went to Mail > Default Addresses in cPanel and set

info@hisdomain.com to be forwarded to info@hisdomain.com
INSTEAD of
info@hisdomain.com to be forwarded to defaultusername

Which was the case by default.

Since I saw a lot of people having problems with Unroutable Addresses (although many seemed to be fixed by the unchecking of not allowing mail to be sent from "nobody") I figured I´d post this here. Big difference here is that mail worked for other customers but not for this specific one. Guess I should have looked in their cPanel in the first place

Hope this may help someone in the future. If you are having other Unroutable Address problems, just fill out those words in the search function of this forum or cpanel.net forums and there should be a solution for you as well