Thanks for your responses. I can now clarify this issue a bit further. I've checked, by sending myself emails, that on my Laptop and Linx device, both running same version and build of Windows 10, are OK re displaying smilies in email, although these have been checked in Windows Mail
So, it's only the PC that has this issue and the same issue is with Thunderbird (default), Outlook and Mail. Therefore, I consider this is just isolated in my PC, and not to any particular email client.
Obviously, this changes things a bit so I'm now looking for the reason for this happening on my PC in general.
Many thanks for all your help. As always, it's very much appreciated :)