Outlook 2010 cannot reply to encrypted email

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http://serverfault.com – A coworker and I occasionally use encrypted email to send passwords. We both are using Outlook 2010, and both of our Digital IDs were created by the same authority. For my coworker, creating, replying, and reading my encrypted emails works just fine. But for me, I can read his encrypted emails, I can send him encrypted emails, but I cannot reply to his encrypted emails. I always get the standard Outlook encryption error message: "Microsoft Outlook had problems encrypting this message because the following recipients had missing or invalid certificates, or conflicting or unsupported encryption (HowTos)