What I would suggest is:
- Install qmail in a test environment in order to become more familiar with it (qmail itself)
- Research and understand the current qmail installation (on Ubuntu)
- Based on the current qmail installation, migration options may present themselves; plan and deploy
It's been a couple years since I deployed or supported qmail. But I can point you to this excellent (free) online book:
Following the installation steps there (on a test
host) really will get you up to speed more quickly. Your post-install configuration, likewise.
With the information you learn from that process, research qmail on the Ubuntu host.
- Where is the qmail installation? What users does it require?
- Is it using any scripts/cronjobs outside of the installation path?
- Where are the mailboxes?
Depending on the answers, it may be as simple as tar-ing up a few directories and scripts, and deploying to the new server. Or, it may be as complicated as installing qmail on the new server, and carefully migrating configuration settings, users, and ancillary programs piecemeal.