[imp] imp and s/mime

[jo256]

i'm investigating a problem with s/mime encrypted mails in imp for
weeks now, not able to find a solution (probably due to a lack of
knowledge about openssl...). hopefully someone can help me.
i'm running imp 4.3 on horde groupware 1.2.3 server (debian etch
32bit). i have imported my private key and have some public keys in
the addressbook. i can successfully sign and encrypt messages. when i
receive a signed message it is successfully checked.
however, when trying to read some s/mime encrypted message imp
"sleeps" for some seconds and then shows me an empty page. in my
apache-log i find the following error-messages:

Error reading S/MIME message
30050:error:0D07207B:asn1 encoding routines:ASN1_get_object:header too
long:asn1_lib.c:150:
30050:error:21078082:PKCS7 routines:B64_READ_PKCS7:decode
error:pk7_mime.c:140:
30050:error:2107A08B:PKCS7 routines:SMIME_read_PKCS7:pkcs7 parse
error:pk7_mime.c:373:

i can only read encrypted messages sent from thunderbird. messages
encrypted in any other mail-client i have tested (outlook 2003/2007,
outlook express, evolution 2.26, entourage) bring the above failure.

my debian-box has openssl 0.9.8c installed.

any ideas whats wrong with my config?

thanks in advance
jo