Moogle Delivery items lost when target mailbox is full

[Cheeto]

So the deal is that I sent 36 items to a friend in 7 sets of 5 and the last alone by Moogle Delivery service. On the 8th mail, the moogle said something about the mailbox being full and the items needing to be returned (or something, I didn't catch it and can't go back). The letter did not appear in my friends inbox, and the last item disappeared from my inventory.

I submitted a ticket, and the response was:
Unfortunately, this type of information is not able to be provided by the Final Fantasy XIV Support Team, since we cannot provide guides and information about in-game mechanics...snip...We Hope you're able to collaborate with other customers to come up with answers...snip

So do any of you other fine customers know what happens when you mail an item to a full inbox and it disappears from both parties?