e.g. errors like
(host #.#.#.51[#.#.#.51]
refused to talk to me: 421 4.4.1 Fatal: Forwarding agent unavailable. Closing connection.
Recycling the DLP email prevent server got the following architecture to finally start working:
Outlook --> (port 25) Ubuntu Postfix MTA --> (port 10025) DLP SMTP Prevent --> (port 25) Ubuntu Postfix MTA --> (port 587, TLS) GMail
The upstream MTA's main.cf was configured with smtp_use_tls=no, smtp_sasl_auth_enable=no; and with relayhost=(DLP_IP):10025
The DLP server's advanced configuration had RequestProcessor.MTAResubmitPort=25 (changed from default 10026). Next Hop Configuration was set to Forward with Disable MX lookup, and downstream local MTA's IP set in Hostnames.