COI sends empty messages to Mail-Client

[matthesatox]

Describe the bug
Messages, sent to a COI user, should also be visible in E-Mail client. At the moment it is the case that the messages received by Mail client are empty.

To Reproduce
Occurrence: always
Steps to reproduce the behavior:

1. Open a new chat or use an existing chat
2. User A sends a message to user B
3. User B receives the message in COI
4. User B opens Mail-Client
5. Check the received message

Expected behavior
Message should also be visible in received mail

Screenshots
If applicable, add screenshots to help explain your problem.

App information (please complete the following information):

• App version: [e.g. 0.4.0]
• Plugin version: [e.g. 0.4.0]
• Delta Chat Core version: [e.g. 0.40.0]
• Flutter version: [e.g. 1.0]

Smartphone (please complete the following information):

• Device: iPhone 11 Pro Max, iPhone SE (4")
• OS: iOS 13.5

Additional context
Add any other context about the problem here.

Internal Bug Tracker.

[Boehrsi]

On non-COI servers this is the case, on COI-enabled servers messages are moved directly. The AppSuite also hides the folders containing chat messages, as far as I remember.

[matthesatox]

I used https://appsuite.beta.coi.me/ for testing and expected that it's just working. It was just working in the past.

[Boehrsi]

Since this backend is COI enabled normally all mails are sorted + filtered (so hidden in the web UI). If they were shown before / are shown right now (empty) this probably wasn't intended.