I don't see how to contact the developer, anybody knows? The privacy policy has email hoa_at_dejalu_dot_me but I don't know if that works, have not received a reply on that. Two issues I observed so far:
1. /private/var/folders/x8 ... I don't know much about schematic of how apps operate on MacOS but I would expect the application to run from the Applications folder. DejaLu seems is copying the DejaLu.app into the folder /private/var/folders/x8 and it runs from there. The issue is it seems to create a new fresh copy of itself every time the application runs? Is this necessary? Is this how applications work on MacOS? The issue with it is with LittleSnitch. Every time DejaLu.app starts, it LS will ask me for permissions, because the location of the DejaLu.app changes in that folder /private/var/folders/x8, every time the app starts, the location changes. The new location is in a virtual volume and the name of that volume is a random string and the string changes every time. So it is impossible to save the LS rule for the application as it is a temporary location.
2. Adding additional email beyond the 1st email is not possible for writing emails. The app complains the Drafts folder needs to be created. Where? In my Outlook account, there is already Drafts. I tried to use Labels to create a label called Drafts. When I tried that to do for the account that doesn't have the folder, the app would just crash. So while it is possible to read emails from multiple accounts, not possible to write email from any additional account beyond the first account that was added.