The level 30 mail restriction was implemented into the game a while back (possibly early to mid 2012). It's to prevent scammers and gold sellers from selling gold and items out to the general population, if I remember correctly.
On a side note, a person can't use the Auction House until they reach 30 as well, unless this changed and I wasn't aware. A lot of things in this game start at 30; Mail, TW, AH, Racial Crafting, 3v3, and MBA are the first ones to come to mind, there might be some others I just can't remember at the moment. 30 seems to be the lucky number for this game.
It's not too hard to get a mule up to 30, it can be done in a day or over the weekend. I'm a casual player, I can't play the game everyday like I could during summer break. As things are now, I only get to play from Fri-Sun. Yet, I still think it doesn't take very long, just the weekend is all. So personally I don't see anything wrong with the level 30 restriction. I'm rather glad for it, like the person above me stated. It keeps the gold sellers to a minimum compared to how badly the game use to be flooded.
As for why the devs have it so a person can't send mail to their own account, when their own character is under 30 to another character of the same account, because the devs don't have it programmed so we can. You'd have to take it up with X-Legends.
That's what the Archive is for; to pass items between characters of the same account, however, not everything is archivable sadly.
This probably doesn't help you much, but I don't know what else to tell you except level up, so you can use the items from your alt again.