When I try to send off any amount of something to someone it shouldn't load the inconvenience of the recipient having a small mailbox to me. Camping the mb is ultimately easier than sending stuff so it becomes way harder for the sender as the amount rises and not as much for the receiver.
What I propose for this is the sender to be able to send off any amount of stuff (capped by the receiver's inventory space limit) and the receiver to camp the mb as before, emptying their mb as it gets filled. To prevent pranks like filling someone's mb with trash a simple confirmation could be sent to the receiver like "XXXXXX is trying to send you 8000 apples, do you accept?" and to prevent it being used like a makeshift bank you won't be able to accept any new trades before emptying your whole mb from the last trade.
