Computer doesn't recognize Developer

Since upgrading to Mojave, each time that I open Mail, I get the window asking that SpamSieve is an app downloaded from the internet, and do I want to open it? I’ve looked under System Preferences > Security and Privacy > Privacy and “Allow apps downloaded from App Store and Identified Developers” is checked. How do I designate that SpamSieve is from an identified developer?

Thanks.

You can’t; that needs to be built into the app. I suspect that the file that you have is damaged somehow. It should help to download a fresh copy.