I am not sure the entire process to achive the goal, but I think modifying zimbraSmtpPort will help you.
su - zimbra
zmprov gs `zmhostname` |grep zimbraSmtpPort
You can modify it with:
zmprov ms `zmhostname` zimbraSmtpPort "<value>"
Or empty it:
zmprov ms `zmhostname` zimbraSmtpPort ""
zmcontrol stop
zmcontrol start |