Removing metadata when sending e-mails (headers)

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**
Metadata's not so nice. The less metadata is generated in such a sensitive context as mail, the better the user can be protected.

**Describe the solution you'd like**
Before the email is sent, all (headers) metadata is removed, as far as this is reasonable and technically possible.

**Existing work**
I know that for example the email provider Tutanota offers this feature and that they also realize it via their clients. Unfortunately I don't know the technology well enough yet, so some research might be necessary to realize this feature.