Well this takes the meaning of despicable into new heights, given how any allegation towards this tech giant can lead you to being financially suffocated if you exclusively depend on their platform.
It is beyond pointless to sue Apple for copying you and they know that BlueMail depends on them on the App Store and also know that they will be embarrassingly reduced to begging them (Apple) to go back on the App Store to survive.
> Without a presence on the Mac App Store, we cannot continue forever,"
As far as Mac Developers are concerned, they can distribute without the Mac App Store, if they don't use in-app purchases. But for Mac/iOS-only app companies who use In-App Purchases however have to ultimately sign an unavoidable deal with the devil and your better hope that he doesn't take your idea soon!