I have a problem with Outlook 2010. When I try to send a mail with digital signature, Outlook sends the mail twice in a row. Both mails look identical and both have the smime.p7s attachment.
When I don't use digital signature, the mail is sent only once. I'm signing mails using my ID card. The certificates for signing and encryption appear in windows certificate store and then I can select them using Outlooks Trust Center-> e-mail security.