They definitely are chained, if I disable the "Retrieve mail for external accounts", the mail module is being reported as Started, correctly. Now I need to understand why the fetchmail is being stopped, I just found a cryptic error message, but to me looks not really useful, anyay, as a reference, here it is (it appears in syslog whenever I try to enable the "Retrieve mail for external accounts".
Nov 13 23:44:30 zentyal4 systemd[1]: Started Zentyal fetchmail daemon.
Nov 13 23:44:30 zentyal4 systemd[1]: zentyal.fetchmail.service: Main process exited, code=exited, status=1/FAILURE
Nov 13 23:44:30 zentyal4 systemd[1]: zentyal.fetchmail.service: Unit entered failed state.
Nov 13 23:44:30 zentyal4 systemd[1]: zentyal.fetchmail.service: Failed with result 'exit-code'.
Nov 13 23:44:30 zentyal4 systemd[1]: zentyal.fetchmail.service: Service hold-off time over, scheduling restart.
Nov 13 23:44:30 zentyal4 systemd[1]: Stopped Zentyal fetchmail daemon.
Nov 13 23:44:30 zentyal4 systemd[1]: zentyal.fetchmail.service: Start request repeated too quickly.
Nov 13 23:44:30 zentyal4 systemd[1]: Failed to start Zentyal fetchmail daemon.