Is kolab srewing up other people's gnupg signatures?

[Johannes Graumann]

Hi,

I have a somewhat convoluted mail setup:
1. A number of mail accounts all forewarding to a gmail account
2. fetchmail imap-pulling the messages from the gmail account and inserting
it into a local kolab installation
3. kontact imap-retrieves the mail from kolab.

Recently - not really sure since when - I started noticing that while a
self-addressed gpg-signed message validates perfectly well from the "Sent
Mail" folder, the received one fails the validation. I can not formally
exclude that this is happening on the non-kolab smtp I'm using, but it's
unlikely since 2 independent smtp servers at my disposal result in the
validation-failure.
When diffing the complete exported mime messages - the sent versus the
received version - I get the following:
# diff Message_Sent Message_Received 
0a1
> 
2,3c3
< Content-Type: text/plain;
<   charset="us-ascii"
---
> Content-Type: text/plain; charset="us-ascii"
21c21
< Content-Type: application/pgp-signature; name=signature.asc 
---
> Content-Type: application/pgp-signature; name=signature.asc

Something in the kolab suite might/is messing up the message header and
invalidates thereby the signature. How might I track this? Who's a likely
perpetrator? Do other people see this or is it something I messed up in the
configuration?

Thanks for any input,
Joh