Missing Mail with items
I’m not sure if you experienced the same mail issue that I did, but I created a thread explaining the oddity: https://forum-en.gw2archive.eu/forum/support/bugs/In-Game-Mail-Issue-Recipient-name-changes-when-sending-items/first#post16307
tl;dr If you right clicked and chose ‘mail item to…’ from your inventory after typing your friends name into the ‘To’ field on the compose mail screen, it is possible that a player name in your characters line of site populated in their place. If that’s the case, it would be on the honor system for the recipient to mail the items back to you.
Cheers!
Was his mailbox full? As far as I can tell, without confirming on my own character, ten items in your mailbox is full.
+1 to what StarsLife said, hopefully that’s the culprit.