Currently I’m using Thunderbird on both my Windows laptop and Apple iMac. I have my “local folders” stored on Dropbox, which allows me to have access to the emails in those folders from either platform. Is it possible to do something similar with eM Client?
No, it is not possible to store eM Client’s database on a cloud synced drive.
The solution, though, is to store the messages in IMAP or Exchange server folders, that way they will be available on any device that connects to that account.