I didn’t see this anywhere online, but managed to figure it out. My new mail gateway was throwing the error:

Aug 17 11:49:28 mercury postfix/qmgr[28567]: warning: connect to transport transport: No such file or directory

I realized it was because I was using /etc/postfix/transport to forward mail for my domains into the exchange server, and had a default rule of “* transport:nexthop” which should have been “* smtp:nexthop” but I misread the manual page.