Linefeeds in Email Headers

[eluser]webthink[/eluser]
We've ran into some problems with Qmail not sending emails because of linefeeds in headers. Changing the linefeed to "\r\n" helped, but we then ran into some problems with bare LFs in the body (Yahoo, among others, rejected the email).

More tweaking, and re-reading RFC822, and we've got qmail sending, yahoo and almost everyone else receiving.

Then we find some users are still not getting email:

"This Email scanner intercepted it and stopped the entire message
reaching its destination.

The policy-violation was reported to be:

Disallowed breakage found in header name - not valid email
"

As far as I can tell, we can't do anything. If we change the LF's in the header, then qmail won't send. It is the MTA on the user's end that is wrong.

Does anyone have any insight into this issue?