Mail preferences window extremely large

I updated my iMac to Mojave and re-installed SpamSieve. When I open the preference window in the Mail program it is horizontally wide, covering the entire screen of my iMac. Further, cannot be adjusted. I called Apple and they suggest it is a plugin causing this. Could it be SpamSieve?
Thanks for any suggestions.

— Richard

I have seen other reports of this, but it doesn’t seem to actually be related to SpamSieve and can occur even without SpamSieve installed. Still, it’s a good first step to temporarily disable any plug-ins using the Manage Plug-Ins… button in the General preferences and see if that makes a difference. SpamSieve doesn’t modify Mail’s preferences window, but some other plug-ins do.

Michael, if I disable the plugin is that sufficient to test it? Or do I need to completely uninstall it first to accurately evaluate?
Thanks,
— Richard

Yes. Without the plug-in, SpamSieve won’t do anything with Mail, and the same should be true of any other plug-ins you may have.