I used the “MailPluginFix” app from http://code2k.net to allow MailUnreadMenu to work (which it now does). But shortly after, SpamSieve 2.9.13 would not work: it slowed Apple Mail down so much that I had to delete the program and all its entrails from my system.
Was it the result of installing the “MailPluginFix” app? If so, would SpamSieve work if I installed it AFTER I installed "“MailPluginFix” app? Or are they just not compatible? Or, perhaps it was just SpamSieve?