catalinamacosmail.app
Since upgrading to macOS 10.15, the mail app is no longer able to open a message by its URL.
For instance, the URL message:%3CCAEkAa-anCn=ZE21kpFw-oKaXEVsGE0SuuNtyUT8qyZndoPMuaQ@mail.gmail.com%3E which launched the corresponding message is now causing Mail app to show the following message:
The operation couldn’t be completed. (MCMailErrorDomain error 1030.)
Mail was unable to open the URL “message:%3CCAEkAa-anCn=ZE21kpFw-oKaXEVsGE0SuuNtyUT8qyZndoPMuaQ@mail.gmail.com%3E”.
My recommendation is to use Apple Mail's rules to customize how you want these displayed—you can make them stand out even more than a simple !! in the message list by setting up a Rule in Preferences:
In this case, specify that you want to apply the rule to messages with high priority. Then choose the formatting that you want. I chose to list messages with a red background. When new messages arrive with a high priority, they got a red background in the message list.
I've had the same issue and found that I've had an incompatible version of GPGMail installed. Removing the files in /Library/Mail/Bundles fixed it for me.
Best Answer
According to your error message it looks like a Mail plugin incompatibility. One or more plugins are not fully compatible with macOS 10.15 Catalina, yet. Did you install GPGMail.mailbundle or additional mail plugins? If so,
~/Library/Mail/Bundles