Exporting and importing an email adds an empty line above the Content-Type header. And trying to import the email again will result in everything including and below the Content-Type line to be imported as the mail body.

Steps to introduce:

1. Export an email.
2. Import the exported email.
3. Export the newly imported email.
4. Open the .eml using a text editor, see there's an empty line above the Content-Type header.
5. Import the newly exported email or open it using Thunderbird, see that everything including and below the Content-Type line is treated as the mail body.

Additional context:
Removing the empty line will allow the mail to be imported and opened in Thunderbird correctly.

Test notes

• Issue no longer reproduces.
• Import-export-import works as expected.
  • For internal mails.
  • For mails sent by an external address to a Tuta address.
  • For mails sent by an external address to an external address.