I just tried to set up exactly this to use my internal mailcow server.
I always got:
REDACTED postfix/error[3029476]: 40F413A0BB5: to=<admin@REDACTED>, relay=none, delay=0.04, delays=0.03/0/0/0.01, dsn=5.0.0, status=bounced (REDACTED:587)
What fixed it in the end was adding default_transport =...