Bad news: In iOS 8, certificates sent in a mobile profile are not automatically installed for encryption. While the own private key is properly used, you will not be able to encrypt for a recipient or even yourself.
There is a workaround: Get a signed mail from your partner, tap on the sender name or address in this mail, and you get a dialog where you can install the certificate manually. Now you can encrypt for this partner. Apple provides a step-by-step guide in http://support.apple.com/kb/HT4979; don´t be confused by the outdated UI, it´s still working.