Re: #121812

Hello @basbloemsaat,

I've spent the day reading through the email module, and RFCs, and I believe I found a better place to fix the issue.
This involved lots of experimentation, so I'm sending an alternative PR rather than a review on yours.

• The generator (writer) verifies that the representation of each header is sound (a parser won't treat it as multiple headers, start-of-body, or part of another header). That should cover custom fold() implementations or Header subclasses.

  • However, some user out there is probably misusing such header injection in working code, so, I added a policy attribute to turn it back.

• Newlines are encoded in fold(), just like undecodable bytes and other special characters.

Overall, this means that we treat newlines as valid content of headers, but “escape” them when such a header is serialized to text.

This PR is a proof of concept. It needs tests and documentation, but I'm out of time for today, and I wanted to share what I have.

Does this look reasonable to you?

• Issue: email.policy.default - gotcha with re-using parsed headers with embedded newlines #121650