If you want to send a unencrypted message to a specific recipient with PGP Desktop (standalone) installed, use the following steps.
This is accomplished by creating a new PGP Messaging rule for the recipient. See the example below:
1) Open PGP Desktop > PGP Messaging
2) Click on Edit Policy > New Policy
3) Create a New Policy specifying a specific domain
4) Click OK and Done
5) Make sure that the new policy is at the top