late response but just letting ppl know in case they read this :P

The mailbox works PERFECTLY I've been using this mod since 2017 still works perfect. BUT the mailbox will let you upload creature that is uploadable but if the creature is modded in anyway you can't leave it in there unless your transferring it and the server youre going to is right after you log off, and has the same mod that contains the dino or it will grey out in the mailbox and be able to pull it out, so if your server has breedable creature bundle that edits the ID of some vanilla dinos like wyverns, rock drakes, beetles, achatinas, rock golems etc cause they arent breedable in vanilla so they are now a modded dino, you can upload anything after taming it and download it before you log off, you just can't leave modded dinos in the mailbox long term. As for vanilla dinos they can stay in there indefinitely so long as you don't change hard drives cause its locally stored on your computer. I have about 500 vanilla dinos in my mailbox and some have been in there over a year and can still pull them out and they're fine lol