I have o365/Exchange configured as an internal relay, so it sends incoming mail for which there is no mailbox onwards to a smart host. The smart host only handles email for addresses prefixed with "otm-".
However the problem is that NDRs aren't generated in this configuration.
I have tried to solve this by making o365/Exchange authoritative for the domain and configuring a rule to redirect only incoming email with the "otm-" prefix in the recipient address to the smart host but I haven't been able to get this right — now everything except the main o365/Exchange mailboxes gets an NDR.
I think I have the rule correct but I think emails are being rejected before the rule is processed.
How should I go about configuring this?