[Client-to-Server] store Emails

[Jonas-Karl]

Description As a user I want access to my received and send Emails. This can be achieved by adding a Datastorage which stores the .mbox files.

Ready Criteria

• [ ] everyone understood the scope of this ticket
• [ ] effects in domain- / useCase- / external-layers are discussed
• [ ] atleast one person is assigned
• [x] mbox file parser exists
• [x] mbox file creator exists

Acceptance Criteria

• [ ] newly send Emails (received in Client-to-Server context) are stored inside a .mbox file
• [ ] old emails do not get replaced
• [ ] good case tests are written
• [ ] bad case tests are written

Scope ✅ Local file storage ❌ Database

Additional context Add any other context or screenshots about the feature request here.