To enable TLS on MAC, please use the following guide:

Kindly note the following guide is base on Macintosh OS X 10.4 using Apple Mail 2. Kindly note for newer Apple Mail version, the location of the mentioned function may be different.

If you are using other mail software, kindly seek assistance from the provider of the software.

1. Launch Mail.
2. Mail > Preferences.

3. On "Account" tab and on the left select the email account that you intend to set the TLS. Click on "Server Settings" under "Outgoing Mail Server (SMTP)".

 
4. Tick the checkbox for "Use Secure Socket Layer (SSL)" and kindly ensure the value is either "25" or "587" and click on "OK" button. 

Once you have set the TLS, kindly try to send and receive again.