Courier seems to expand all external addresses in aliases so that the DSN value is set to "NEVER" (NN in the control file). It'll then deliver the message to the external address with either an empty return path, or NOTIFY=NEVER, depending on whether or not the remote server supports the DSN extension.
Is this desirable?
Yes. As far as the sender is concerned, the message has been delivered to the recipient. The sender does not care that one of the recipient addresses is an alias, which then forwards the E-mil to a nonexistent address that bounces.
What if the destination is an alias on a remote server? Will that not prevent users from seeing delivery failures that occur after the first hop?
Precisely. I can't tell you how many times some idiot sets up a forward to a full mailbox, and I get the totally useless bounce, as far as I'm concerned.
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