@Schlicki2808 Ok, I can try to explain my thoughts regarding a dedicated background service/daemon, but I might be horribly wrong on some or all of it as I am just an amateur. Anyone, don't hesitate to correct me in that case.
After the latest release which features database encryption, it's quite easy: A daemon which is started automatically does not know the database passphrase, so it won't work before you start the app for the first time after booting. Using a keyring would solve this, but to my knowledge, there's no keyring in Ubuntu Touch at the moment that is automatically unlocked along with the device. In addition, users might want to use a stronger password for the database than the unlock PIN.
Even if database encryption is not active, it's still more difficult to implement than a daemon for Dekko because Dekko doesn't support encrypted emails. In contrast, encrypted messages are the default in Delta Chat/DeltaTouch. So the daemon would have to be able to read the database on the device, talk to email servers and decrypt the messages (to be able to show a part of the message in the notification). In my app, I did not program these tasks myself. Instead, the app uses libdeltachat (deltachat-core-rust) by the developers of Delta Chat. I just added the GUI and a little bit of logic. I don't have the skills and the time to re-write all of this just for a daemon, so it would make sense for a daemon to use libdeltachat as well. However, I think that libdeltachat doesn't have a read-only mode for accessing the database (again, I might be wrong on this). So when the app starts, the daemon would have to drop the database (easy), and when the app is closed, take it up again (no idea how to reliably inform the daemon that the app has closed).
Another possibility would be that the app uses the daemon to access the database. At the moment, that would require an extensive interface. In the future, however, libdeltachat functions will be accessed via JSON-RPC, so an inter-process communication would be a lot easier. I'm not sure though whether such a setup would be fast enough.
What's more, apps that run a daemon had to be unconfined in the past. Probably this will change (or has it already?) with focal though.