S/MIME with CAPI

I'd need to handle S/MIME mails using CAPI.
AFAIK there is all needed for that in CAPI but I'm not able to make it work.
At first, I'm trying to validate p7s signature from mail created in
Thunderbird but with no success.
I tried CryptVerifyDetachedMessageSignature which always returns false
so I either send there wrong data for validation or (as I read
somewhere) it uses MD5 instead of SHA1.

In the next steps I'll need to create p7s signature and later on
crypt/decrypt p7m mimes.

Please can someone direct me to functions I should use to accomplish
that? Unfotrunately, "googling" hasn't helped me much.

Thank you,
Petr Langer
.