Due to the recent version changes, cPanel software uses a secure version of TLS, v1.2 and will prevent email clients from accessing secure email with older, less secure ones such as TLS v.1.1 , TLS v.1.0 and SSL 2 and 3 . It is possible that the version of the mail client being used or the version of windows is the major reason for the recent issues. For example, if on Windows 7, Windows 7 may not be fully up to date or may not offer appropriate support for the minimum required version of TLS cpanel now support. Microsoft itself has published methods of allowing this to work:
It is better to try configuring your operating systems and web browsers to use TLS v1.2 according to sites such as these:
Follow the instructions below:
a) Update Windows 7/8 using Windows Update and download/install all important updates. For windows 7 is important that you install Service Pack 1.
b) Install KB3140245 and reboot: http://catalog.update.microsoft.com/v7/site/search.aspx?q=kb3140245
c) Execute the EasyFix and reboot: https://support.microsoft.com/en-us/help/3140245/update-to-enable-tls-1-1-and-tls-1-2-as-a-default-secure-protocols-in
d) Start > Search for Execute, Type Regedit and press Enter to access Windows Registry.
Check this registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\Schannel\Protocols
Under the Protocols key, add two new keys, if not already there: One called “TLS 1.1” and one called “TLS 1.2“. Inside both of these keys, add another key called “Client“. Now create a DWORD value in each Client key called “DisabledByDefault” whose value is 00000000.
Reboot and start Outlook.
If Outlook is still not able to send or can't connect to port 465, change to port 25.
Do check the url at: