Most Elegant Way to Sync Virtualman Mailboxes with an IMAP Server?

[calport]

@RJM_Web_Design

The site @calport sent me to also worked using a Web-based app that he makes available for free with a 3GB limit (but no limit if you pay for a license). It doesn’t actually maintain synchronization, but it synchronizes changes since the last run. That would also work for my use case.

Yes, and if you install the FOSS Imapsync on one of your servers then you could use CLI and cron to transfer email very fast and without the issues you experience in this instance when you use rsync et al. Though I must add that I did use the rsync method a few days ago to transfer over 200 GB of email from Virtualmin on Debian 9 to Virtualmin on Debian 10 quite successfully.