Separate iMessages on iPhone and Mac
What's happening is iCloud is syncing your message settings across devices. You'll need separate accounts to accomplish this.

You will need to first remove this email from your AppleID (presuming it's an alternative email on your AppleID). 

Create an iCloud account for your Mac using just your email and log into that on iMessage on your Mac. This way you have a totally disconnected account and changing the settings won't effect all of your iCloud-connected devices on your first account.