I love having my rather large email accounts backed up to, and searchable, in DTP. This feature is a killer feature.
However, in its current form, it’s almost unusable. I’ve tried importing from both the Mail application and Outlook, and in both cases, it is slow to import or update, and crash-prone. I ended up writing software to break apart large folders into lots of smaller ones to prevent it from crashing. That change has turned my folder landscape into a bit of a mess (not to mention it took many hours to get all this to work. The code is here if you’re interested, though it’s not for the faint-of-heart: https://github.com/ebowman/imap-filer).
What would seem like a vastly preferable implementation, would be for DTP to just use IMAP to talk directly to the email server. The creators of the IMAP proposal designed it for exactly this use case: it supports incremental updates to a caching client, which DTP is. There is no need to be able to send emails as well through DTP, I get that it’s not an email client, but this would be a vastly preferable implementation for saving emails to DT3. I suspect it would also allow getting rid of some code in DTP that has never really worked that well anyhow.