I just upgraded to Mojave from Sierra. After the upgrade Mail would launch, open the menu bar with almost all options grayed out, and just sit. No windows, no options, no preferences.
After poking around and progressively moving prefs and account storage from the user Library I discovered that if the Bundles folder and the PersistenceInfo.plist from the Mail directory were removed, Mail would launch correctly.
I went through the SpamSieve install instructions, downloaded the latest version and made sure that the SpamSieve had full disk access before I tried to install the Apple Mail plugin.
As soon as the plugin is installed, the Mail app breaks again. Same issues.
Suggestions Michael? It’s only happened on this one machine. Another had no problems after Mojave upgrade from High Sierra.