MIDI output not sent to external MIDI port (Windows)

If you are experiencing issues with MIDI output not being sent to an external MIDI port (MIDI interface hardware), please check the following:

1. Make sure no other programs besides Synfire are connecting to the particular MIDI port at the same time. There is a limitation with some USB MIDI drivers on Windows that doesn't allow multiple programs to use the same port at the same time (lack of multi-client capability). So, if possible, make your DAW listen to (or send to) a different MIDI port.
2. Some MIDI interface drivers don't even allow a port to connect to one program for input and to another program for output. In this case, be sure you disable the "Input" tick box for that port in Synfire, if you want to also use it for output. This is because Synfire and Audio Engine are separate programs which, only in this case, cannot connect to the same port for input and output at the same time. 
3. If that doesn't fix it, please disable "Use Audio Engine for MIDI Input". This will make sure only Synfire is connecting the the MIDI port for both input and output.

For further information, also read this solution.