Please tell me if/when this is implemented – I’ll buy it immediately 
The problem
HTML emails that are backed up will become useless in a few years due to leak links for the embedded images.
Proposed solution
New setting to store loaded images (and external media) to a separate database, which will serve as a fallback when viewing the email in offline mode in the future. The database should just be a URL-to-binary sqlite.
Settings:
- Off (default)
- Automatically save loaded images to database
- Manually save loaded images to database
- User presses a button in the context menu to stash all loaded images in the current email to the database.