Thanks QwerkyPengwen, but I really didn't want my address list/contacts on a cloud server. And I can't find anywhere in Windows Mail to simply import contacts from a local file on my computer. But I may just be dense.
Forgive me, but your last post said Gmail. So that's what I gave you.
If you don't want to deal with the cloud, then Windows 10 mail app isn't for you, because for contacts, you have to use the "people" list thing, which requires you to do it in the website which is cloud based.
Any particular reason why you don't want to use thunderbird anymore?
In any case, your only options are to use 3rd party applications that you have to download and install just like with thunderbird.