Simon Ser

Yes. I'd rather not parse these broken messages.

Yeah. I agree the error message should be more descriptive.

I'd just say this e-mail is _too_ broken.

- Should this be tailored for emails, or for MIME messages in general? - Should we ensure reading a message and writing it back will yield a byte-identical result? (I...

On that note, maybe we should introduce a `WriterTo` interface. This could allow e.g. go-smtp to use 8-bit encodings if the SMTP server supports 8BITMIME.

This doesn't work if `Entity.Body` is set to something else by the library user.

I've reworded this to avoid using the word "default".

> Though I am not sure this is correct in the presence of a bouncer. Bouncers would reply to PING with their own time, while TIME would be forwarded to...

It doesn't seem like these commands are widely supported, e.g. `SQUERY` on OFTC gives "unknown command".

Nice! I think to fold `h`, it'll probably be simpler to do it before turning the list into a string, ie. in `formatTagList`: https://github.com/emersion/go-msgauth/blob/fb07ec9e7323788ae953eea6797a1dddb8a13106/dkim/sign.go#L340