You may run into connectivity issues if you have any of the following mail client software installed on your local machine:

• Windows 7 (you can manually enable TLS 1.2 which should fix the problem, see notes at bottom)  
• Windows XP and older
• Apple Mail 9.3 and older
• Mac OS 10.11 (El Capitan) and older
• Mac OS Mojave Version & Mail App 12.2
  (may require manual port settings)
• OEClassic
• Eudora
• Microsoft Entourage (all versions)
• Outlook for Mac 2012 and older
• Thunderbird 27 and older
• Windows Live Mail

Note:

Outlook 2007 and newer work fine on Windows 10

Windows 7 - How to Enable TLS 1.2

1. Click the Windows Button in the lower left hand corner (standard configuration) of your Desktop.

2. Type "Internet Options" and select Internet Options from the list.

3. Click on the Advanced tab and from there scroll down to the very bottom. If TLS 1.2 is checked you are already all set. If it is not, please check the box adjacent to Use TLS 1.2 and then Apply.