While I don't necessarily believe that SE nixed mail between characters on one account because they planned on implementing monthly-fee retainers, now that they HAVE monthly-fee retainers that's pretty much the nail in the coffin when it comes to the idea of ever freeing up mail.

I think we can all agree that being able to mail stuff to other retainers on your account pretty much invalidates the need to buy extra retainers. If you don't have enough space, just make another character and send the extra junk there! So, if they were to implement it now, it would be a slap in the face for folks who pay for retainers now AND would result in pretty much everyone with extra retainers cancelling them immediately.