I wanted to email a player to thank them for a great map they had created
But their name is not in the list
In case it makes a difference they are French and were last in warzone about 90 days ago

If you wish to email a player from the https://www.warzone.com/Discussion/SendMail page, they need to be on your friends list. If you have their profilelink, you can mail anyone directly from there using the Send Mail button.

But their name is not in the list on that page
Loads of others whom I have no idea who they are are on the list

when you start playing warzone there is a checkbox that adds everyone you played as your friend. I'd suggest you uncheck it.
And yeah, find the person's profile, add friend then type their name into send mail and you should see it
Or, when you're on their profile to add them as a friend, just send mail : ))
Same on app as browser

Good luck!