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This is a Hint-n-Tip post.
3CX documentation confirms that Google Cloud compute-engine hosts will NOT be able to send outbound mail on any of the traditional SMTP ports, esp not port 25 which is what 3CX expects to use.
I've set up my firewall at the office to accept email on port 26. (well, technically it accepts traffic on port 26 and passes it to port 25 of the mail server, which never knows anything odd happened).
I was playing with IPTables on the Google hosted Debian to re-route outbound port 25 traffic to port 26 but couldn't get it to behave. Then I found a hidden setting in the 3CX PARAMETERS table which tells 3CX to use port 26 to start with. The parameter is
MLSRVPORT
3CX documentation confirms that Google Cloud compute-engine hosts will NOT be able to send outbound mail on any of the traditional SMTP ports, esp not port 25 which is what 3CX expects to use.
I've set up my firewall at the office to accept email on port 26. (well, technically it accepts traffic on port 26 and passes it to port 25 of the mail server, which never knows anything odd happened).
I was playing with IPTables on the Google hosted Debian to re-route outbound port 25 traffic to port 26 but couldn't get it to behave. Then I found a hidden setting in the 3CX PARAMETERS table which tells 3CX to use port 26 to start with. The parameter is
MLSRVPORT