We have seen a number of times when mail between avatars that are offline with attached inventory items (for the owner of those items) fail to arrive. I think I pinned down at least one case where this happens. I sent 3 messages with the usual single inventory item attached to each for an item my avatar owned. I did this to two other avatars. The first logged on and the 3 messages were there. The second avatar logged on and the messages were not there. The items had to be sent again. There may be some glitch related to only a single copy of an items (even where the owning avatar can send them fine) being "in transit" perhaps? I have seen this issue on both the Live server and , as reported in this observed case, on creator.sine.space.