I have been using SpamSieve in conjuction with Apple Mail on my MacBook Pro for years (on several generations of laptops) and loving it. For some reason, when I was updating the program today, as I went to reopen the program, it would not open. Not a blink, flash or anything. It just won’t open. A popup reported a “Sandbox error” and suggested I double click the icon in the finder, but this did absolutely nothing as well.
Please try installing a fresh copy of the app.