Hello Thugor,
Under the PGP Messenging Tab, Highlight the policy and click on Edit Policy.
At the bottom of the Policy, change if no result to Send Message unsecured.
This will stop the blocking of the e-mail message.
Also, you can change the order of the policy. Using Opportunistic policy first will search for a recipient's key and if not found the message will be sent in the clear unsecured.
Thanks
Anthony