said by Curmudgeon:
I agree with jmcneill. That is the behaviour I would expect. It's more secure. I did something similar with my e-mail and I just left the forwarding (or fetching) in place indefinitely. No reason to remove the address.
+1, this is expected behaviour from some mail servers (although one might argue that mail shouldn't be accepted for non-existent accounts anyways, but there's no arguing with how TSI has it implemented).