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