Windows Mail App Transfer

[joe43wv]

I have a laptop that has an email account setup in Window Mail app. The customer had old emails from an old laptop that they wanted  transferred to the new laptop. I have the emails from the old machine but I cannot import the old messages into the mail app. I've downloaded Thunderbird and I know how to transfer the emails from the old laptop to Thunderbird, but where I am stuck is transferring the emails from the Windows Mail App to Thunderbird. Can anyone point me in the direction of where to find the location of the emails and/or get those emails to transfer into Thunderbird?