I kind of think they can, but Im not sure if it's still of the 'I accidentally deleted the item' kind rather than 'I just let the mail get deleted', which doesn't make sense to me if the guy never got them...they sit there in the mailbox for an entire month, that's way longer than the week he says he lets them sit for rested.

That particular function of stable masters was put in back when the achievement system was patched into the game and -all freakin hell- broke loose, servers reset randomly and on purpose and if you had tried to learn your mounts/minipets so they weren't items in your bags anymore before one of those resets, you lost them. Blizz eventually shook that out and the stable masters were able to return mounts and minipets by giving you copies of everything you had.

I seriously almost quit the game when all of that was happening. I was a collector even back then, despite the hell it wreaked on my bank and inventory (And I was a hunter, I lost a bag slot to a stupid quiver!) and to lose the mounts I had worked so damn hard for, because it was still hard back then to get exalted with other races, was devastating. My girlfriend talked me into waiting, sure Blizzard would figure out a way to fix it, and thankfully they did.

Stablemasters can help you retrieve some pets you once had, but no longer have. Specifically, they are supposed to be able to help you reacquire pets that were originally soulbound items that you possessed but lost when pets were converted to the spell book. If you never removed the pets from the mail, then your character never had them bound, and the stablemaster won't be able to help.

Not that they can anyway. Several of my toons are missing their corehound pups and stablemasters offer no assistance, and I cba to fill out a ticket for them.