Server
Take the whole instance with you, or bring one domain in from another SelfPost. Both files are secrets.
data/ (database, DKIM keys, credentials), docker-compose.yml, .env and certs/ in one archive, without the Postfix queue and without a certificate file the panel may not read. Restore into an empty project directory and start the same SelfPost version. The reverse-proxy vhost is not included.
A domain export from another instance — plain .json or encrypted .spde. Its DKIM key and application passwords come across, so the published DNS needs no change. The export itself is made on the domain's settings.