+1

Shameless bump.  I still have to regularly clear out system notifications and NDRs from my spool.

We're in the same boat. I already reported this months(years?) ago without success. NDRs filling the spool because they're not relayed through our outgoing gateways.

At some point in previous versions, before one of the "big" release, it was using the gateways for NDRs and everything was ruinning smooth. Then one day, after an update, I noticed it started bybassing the gateway... :(

While troubleshooting this problem, I found that system messages are attempting to deliver to my domain's MX servers.  But, for some reason, the connection dies during the SMTP session.  Because the cipher is ECDHE, I am unable to decrypt the session in Wireshark.  Before getting more fancy with the process, Jereming recommended that I change the SMTP Out binding under Settings>Protocols to use the Internet-facing NIC.  Doing so fixed the issue with delivering system messages, but it broke mail flow for gateways on the internal network NIC.

The option at this point was obvious: regular client mail flow trumps system messages, so restoring the SMTP Out to the internal NIC and breaking system messages is the going-forward configuration.

This feature request is under consideration, but more "yes, please," would be helpful.  Thus the shameless bump.

Same request for *forwarded* messages.  We found that even when the client domain is set to use a specific outbound gateway, that only works for messages sent by the client.  When a forwarder is set up (eg. forwarding their mailbox go their Gmail or whatever) the gateway setting is ignored and the forward is sent directly without relaying it to the gateway.