One of the first things to check on with system updates is the compatibility of apps you regularly use, as well as any sort of add-on. Apple Mail (or just Mail, or Mail.app, or sometimes Vicky, not that there's anything wrong with that) is made more useful by the addition of tools like Mail Act-On or SpamSieve.
Occasionally a small system update also updates Mail and can break your (sometimes fragile) mail setup. What happens is that there is a version of Mail listed in the plugin and when your version of Mail doesn't match, your plugins are declared incompatible by Mail and then you're stuck.
Or you were, until MailPluginFix came along. This is a utility that shows your "incompatible" plugins and fixes them by updating the version number for you. Generally this is a reasonably safe bet, since changes in Mail tend to be smaller in the incremental releases (say, from 10.10 to 10.10.1). Your best bet is to wait until compatibility is guaranteed by the developer, but if it was a small update from Apple that broke a plugin, this can be an easy way to keep things working until a more official update.