Php mail() with TLS/SSL

I want to use phpList to send mail using the php mail() function (no SMTP) but when I use this GMail says my emails were sent unencrypted. The message is “ did not encrypt this message.” I’m new to phpList and this is my first time setting up phpList. Is it possible to encrypt emails sent using the php mail() function? If so, any advice on how to set this up?