Tracking pixel is added to S/MIME mails breaking the signature

## Describe the bug

When sending a PGP encrypted mail, Postal does not add the tracking pixel. This is also true for PGP signed (not encrypted) mails, probably to keep the signature intact. When signing the message with S/MIME however, the tracking pixel is added, breaking the signature.

## To Reproduce

1. Enable email tracking
2. Send PGP encrypted mail --> tracking pixel is not added
3. Send PGP signed mail --> tracking pixel is not added
4. Send S/MIME signed mail --> **tracking pixel is added**

## Expected behaviour

Nothing should change the mail body when mail is signed with S/MIME.

## Screenshots

S/MIME signed mail with the added tracking pixel and the signature


## Environment details

- OS: Manjaro Linux
- Mail Client: Thunderbird, current version
- Browser: Brave, current version
- Postal Version: 3.3.4

## Additional information/context

It would be interesting if Postal would have S/MIME support by itself, so it can add the tracking pixel and sign the mail afterward. This would also make it possible for transactional senders that do not support S/MIME to sign with S/MIME. However this is not part of this issue, just a proposal.