Enabling Secure Connections in Mac Mail App (prior to High Sierra)
If you are using Mac Mail on OsX High Sierra, it is already using secure ports.
For prior versions: Change your Server Settings to enable Secure SSL encryption for an email account in Mac OS X Mail:
1- Select Mail -> Preferences from the menu in Mac OS X Mail.
2- Go to the Accounts category.
3- Highlight your HIS-hosted email account.
4- Go to the Advanced tab.
5- Make sure the Use SSL checkbox is selected. Clicking it will automatically change the port used to connect to the mail server.
IMAP will use port 993
POP will use port 995
6- Click Server Settings.
7- Click the outgoing Account pop-up menu, then choose Edit SMTP Server List.
8- Select the Outgoing server for your HIS account.
9- Change the SMTP server port to use port 587 with STARTTLS or port 465 with SSL.
11- Click Save.
11- Close the Accounts window.