Windows 7: IE11 no longer supported, along with third party email clients that use it
 

If you still use Windows 7, you have to stop using Internet Explorer (Edge is supported). Many email clients also use IE (mshtml.dll) to display HTML.

Outlook 2016 is probably OK as long as it's supported. Thunderbird uses Gecko (Mozilla). eM Client uses Chromium. Anything else probably uses mshtml, especially if it's less then 40MB. Corrections welcome.

Why do you have to stop using IE 11?
If you are really concerned about security you should move to Win 10. IE11 will remain usable but not supported/updated, same as Win 7.