Adam Sjøgren wrote:
>
> > I did some more systematic tests.  The mangling is visible in the
> > locally saved version of the message, so it's definitely caused
> > locally.
>
> What format do you use for your sent messages folder? nnfolder?

My setup is similar to Eric's.  I use nnimap and the backend is a local
dovecot that is synchronized (but that doesn't matter here) using isync.

> > The problem does *not* occur if I do not sign the message!  It
> > *does* occur when the message is signed either with an OpenPGP or
> > with an S/MIME key.
>
> Aha. Maybe that is to avoid the lines starting with "[Ff]rom " being
> (potentially) mangled after signing, leaving the signature invalid.

Maybe... In order to debug this, it would be useful to examine the
various steps that lie between pressing C-c C-c and storing the
message.  Is "Edebug" the way to go?