I cannot find the location of the test1 mails

[simonladen]

Using mail-tck 2.1.1, I set up the docker image with the mail server (jakartaee/cts-mailserver:0.1), I start the server, create the user, use fpopulate to populate the mailbox with the content of test1.
Then I run the tests, but no matter how I define JAVAMAIL_ROOT_PATH, the tests fail to find test1.
if JAVAMAIL_ROOT_PATH="", I get the error:
[INFO] [exec] [javatest.batch] Exception in thread "main" java.lang.NumberFormatException: For input string: "test1"

if JAVAMAIL_ROOT_PATH="/", I get the error:
[INFO] [exec] [javatest.batch] jakarta.mail.FolderNotFoundException: /.test1 not found

if JAVAMAIL_ROOT_PATH="/root/", I get the error:
[INFO] [exec] [javatest.batch] jakarta.mail.FolderNotFoundException: /root/.test1 not found

Yet fpopulate returned:
Populating test1
Copy from test1 to test1
Copied 25 messages
(I can have similar results with test2 or 3)

Is that path supposed to be in the file system? I cannot find the mails in the container file system.
Connecting to the mailbox using Thunderbird, the inbox appears to be empty.