Disabling PGP decryption in Apple Mail requires deleting a “bundle” file used by the application. Your existing keys will remain available on your machine.
First, click the Mail icon in the dock.
2. Click “Mail” in the menu bar on the top of the screen, and select “Quit Mail.” This is to make sure it’s shut down completely before we continue.
3. Click the Finder icon in the Dock.
4. Click the “Go” menu in the menu bar on the top of the screen, and select “Go to Folder…”
5. This will open the “Go to Folder” window. Type this exact text: /Library/Mail/Bundles
5. At this point, you may see a folder with the “GPGMail.mailbundle” file. (If you don’t, return to step two, and in step 3 instead type exactly ~/Library/Mail/Bundles. You can type the ~ (tilde) character by holding shift and pressing the ` key, located directly below Esc on most keyboards.)
6. Move the file “GPGMail.mailbundle” to the trash, either by dragging it to the trash icon on the dock or by right-clicking it and selecting "Move to Trash."
6. At this point, you may be prompted to type your macOS administrator password. Type it in, and hit the “enter” key.
You may see the file deletion dialogue displayed on the screen.
Once the GPGMail.mailbundle file is in your trash, your emails will not be automatically decrypted in Apple Mail.