I see you posted two questions on the same thread. I am going to answer both of them here.

Trusting SSL certs

Add the following line in the server.properties file:

trust.server.for.starttls=true

Once added, Xeams will trust the self-signed certificate from your other server.

Moving Xeams to Another Server

Check https://www.xeams.com/movexeams.htm for instructions. Xeams does not use the MBOX format. However, copying files from one machine to another will bring the mailbox over. The mailboxes for users are stored in $INSTALL_DIR/UserRepository folder. On QNAP, $INSTALL_DIR should be /share/CACHEDEV1_DATA/.qpkg/Xeams .