Hello and good morning. First, you can upgrade from 15 to 17 without needing to upgrade to any versions in between. I do recommend you complete a full backup of the SmarterMail environment before beginning though, as this will allow you to cleanly roll back IF anything fails to upgrade. However, I can assure you the upgrade process has received a good amount of attention over the last few months, and all but a couple misc environmental issues have since been resolved. If you DO run into upgrade failures I suggest reaching out to support right away so we can assist and determine what inside the domain caused the conversion issues. With that being said, below you will find a backup routine you can use to take a full copy of your existing environment:

Backup SmarterMail (Pre-17): https://portal.smartertools.com/kb/a3189/backup-and-restore-smartermail.aspx

Once backed up, you can ensure a smooth upgrade by preinstalling:

- Windows Updates

- .NET 4.7

- .NET updates (quality rollup from Microsoft has had positive results on a variety of issues.)

After those have been deployed, I suggest first migrating to your new server (without upgrade) on the existing version. Once the domains have been established on the new server with existing version, you can then proceed with an upgrade to the latest release. You can find the latest v15/v16 builds under the Legacy Downloads section of our SmarterMail downloads page. Migrating SmarterMail is documented here: 

Once migrated and up and running under your existing version, use the major version upgrade instructions to complete the upgrade process: 

As always if you get stuck please don't hesitate to reach out to our support team for additional assistance. Please let me know if you have any outstanding questions on this.