I'm probably not alone in discovering that mailing items back and forth makes for unlimited bank space. Or if not permanent, leaving them sitting fora month buys time. At only 30c per slot, it's very cheap, even when going back and forth and back and forth and... You get the picture.
The problem though is that it makes it much harder to find items. Does my bank alt have that or did it bounce back? Or did she send it to someone else?
It can be a bit of work too when an item is going to be deleted soon. I have to pull it out of the mail and then resend it. This also means I have to scroll back a few pages to check all my mail and I need bag space to make the switch, which is the thing I lack that makes me use the mail this way.
I don't actually know, but it might also be harder on the servers than a bank. Rather than remembering item and location, it would also have to remember the item, the location in the letter, the location of the letter, the time it was sent, who sent it, whether it was freshly sent or a return, etc. While this wouldn't need to be called up as often as bag information, it would seem to be more complex.
Then there's the issue of auto-delete. It takes two months, how could that be a problem? Well, wait two months and you'll see. A few weeks ago I started playing a lot less due to a very low care cup. Still, I cared enough to not want everything to get deleted. The result was a struggle to find space, scattering items across characters with a small semblance of reason, making some effectively unplayable due to their inability to loot anything. I also gave away a lot of cobalt since it wasn't selling well.
This inspired me though. I decided I would no longer use the mail as a bank. If I couldn't fit it in my bags somewhere, I'd have to find space by selling, vendoring, or finding someone else that could hold it. I actually made a little bit of gold off this, finally getting rid of some items which I'd not bothered to put on the AH. Not thousands, but maybe a few hundred, maybe a thousand.
I must admit that I am still not entirely successful. My bank alt still has a lot of weapon oils and some other miscellaneous items, sent to me by a friend from, well long story. And I've started using the mail as a vendor for trash. While my paladin and DK were farming Jeeves (my paladin finally made it last night and was disappointed to not have a mailbox) they pulled off a lot of trash from the library guardians. I used MOLL-E to send stacks of trash to my shaman. I suppose this isn't too bad since it's very temporary, getting sold off the next time I log in to my shaman (which these days is more than once a month).
How about you, do you use your mailbox as a bank and vendor?