I just came to read this hoping there was an easier way than what I was doing but nope. I just cleared my box out completely- only the second time I've ever done it. Usually I just delete a few pages to open up some space. Then I deleted all of my sent messages. I have a completely empty, sparkly clean message box!
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226