I have had the same toon to toon mail issue. From reading several other posts, it would appear this is a bug and I hope it gets corrected. If not, there are lots of us who are out items, astral diamonds, and coin. If anyone finds an answer or a fix, please post it for the rest of us.
I tried the delete the old mail trick and it did not work either. I lost about 10 pieces of assorted gear and it looks like I will just be eating the loss. Welcome to being a beta adapter.