PlayMinetestServer/mods/mail_mod/onjoin.lua

minetest.register_on_joinplayer(function(player)
	minetest.after(2, function(name)
		local messages = mail.getMessages(name)

		local unreadcount = 0

		for _, message in pairs(messages) do
			if message.unread then
				unreadcount = unreadcount + 1
			end
		end

		if unreadcount > 0 then
			minetest.chat_send_player(name,
				"(" ..  unreadcount .. ") You have mail! Type /mail to read")

		end
	end, player:get_player_name())
end)