Provide more ways for middleware to interact with mail parts

[wneessen]

Is your feature request related to a problem? Please describe.

For the OpenPGP middleware we are currently working on, we need more ways to interact with mail parts. For PGP/Inline for example, we can only support Plain text mails. Therefore we need a way to delete alternative body parts (like text/html).

This ticket does not yet provide what is all needed, so it will be used as collective issue for all enhancements that are going to be developed.

Describe the solution you'd like

We need ways to interact with/modify body parts.

Describe alternatives you've considered

No response

Additional context

No response