diekuh Thank you!
I restored my files using this docker command:
sudo docker run -it --rm -v mailcowdockerized_vmail-vol-1:/vmail -v /opt/philip-sent:/philip-sent debian:buster-slim
I realized that it is important to preserve the timestamp of the files, so I used cp -a /philip-sent/cur/* .../philip/.Sent/.
Now everything works fine 🙂
P.S.: The permissions are -rw------- (I think it’s 0600)