In 1995, I started using Pegasus Mail. It stores all messages in mbox format. In 2009, I switched to Postbox, which also uses mbox for the local email folders, so importing my email archive went effortless by just copying the mbox files.
Now, after Postbox is end of life, I have to choose a new email client. eM Client looks very promising (and importing my Postbox archive into eM Client worked great), but one thing worries me: there seems to be no way to smoothly moving my local email archive from eM Client to a different email client in the future. Exporting a million of separate .eml files is of course out of the question. IMHO, there should be a way to export the local email folders in a standard file format to be imported into whatever email client I want to move to in the (hopefully far) future. Or instead, documenting the internal eM Client database structure for the local archives will do too, so it will be possible to make your own export script as needed.
As for now, I don’t know what to do next. I like eM Client, but I don’t like the lock in. Any thoughts on this?