For those trying to use this function from Windows with a key in Outlook or Outlook Express it can be tricky to figure out how to get the key exported in the format that OpenSSL is looking for. Since all (at least all Microsoft) products share a common key store, it's easier to export the key from IE than Outlook.
In IE select Tools -> Internet Options, then the "Content" tab, and click the Certificates button. Select your certificate from the list and click the Export button. To encrypt email you only want your public key exported in the "Base-64 encoded X.509 (.CER)" format. The file this procedure creates can be directly used as a key file to S/MIME encrypt with openssl-pkcs7-encrypt.
