Security For email in Virtualmin default installation


I am very new to virtualmin but old in linux/system administration. i noticed that i can create/manage email account in desktop client without encryption for server to email-client and email-client to server communication. is there any particular reason that STARTLS or TLS like protocols are not utilized or this choice has been left to users that they can manually add it later on? if yes, then if you guys can share any helpful howto for this that will be very great and helpful.
Your help/guide will be highly appreciated.