Based on what I read in HGL forums, and my experience with mail in WoW (I will specify below):
1) You can only check the text in your mail while not in a station.
2) While in a station, you will be able to get the attachment from your mail, and send mail with attachment.
3) You can send mail with/withou attachments to any toons in game, whether that toon is online or not.
4) There is going to be an expiration date to all the mails in each toon's mailbox, there maybe a different expiration date whether the mail has attachment or not, and whether the mail has been read or not. (E.g. in WoW, each unread mail has a longer expiration date, 30 days if I remember correctly. Once you opened the mail, it's expiration date will be shortened to a few days.)
5) If it works like WoW, once you attached the item in the mail, it will be in the mail system and not taking up your inventory space. While you can use the mail system as a temporary storage, it can't be permanent since as mentioned above, there will be an expiration date. There may also be a postage fee, even though it may be low.
6) BTW, you will be able to send out a mail to everyone in the guild. One way for KoP to spam us...
7) Consignment House, is unlike Auction House that there will not be any bidding involved. It's more like a market place where you put up your item with a fixed price and when people like the price they can just pay and take it, like the Buy It Now option in Ebay/WoW's AH. However, if it is like WoW/Ebay there may be a fee to put your item up in the CH. This fee may even be proportional to your list price. So if you want to use the CH as a storage, then you probably want to list your item with a very high price to avoid ppl from actually buying it. But then the fee will be higher. You may as well use the mail system. Besides, I doubt you can list your item forever. In WoW, you pay more to have the item listed for a longer time, but at least when you do list an item, it will be taken from your inventory and free up some space.