With the removal of @mail and the mail system as a whole, there's not much that can be done in-game to remedy this. So once again, we look to the website for help. This is a 2-part solution, in which one of those parts are already partially ready to use.
Sending items was a big part of @mail. The ability to send an item to an offline (or AFK) character or someone too busy to come to you right away was a nice benefit. So to remedy this I am proposing that we add a new feature to the Market 2.0/Web Storage. A "Send Item" feature which will allow you to forward items to friends (or your alt accounts). Since Web Storage has no weight limit and can store far more items it's actually in some ways better than @mail.
If someone you send items to is in the game they'll receive a notification in the chat box letting them know they received items into their web storage. There's currently no other way to leave messages for an offline player so I'm looking into this.
Another important, albeit lesser used feature, was sending a message. This already is possible via the community. You can send PMs to people easily via the existing community software, and now that character names are used as display names it's much easier to find your friends and know who you are really talking to. This has the upside that you can even send messages while offline, but the downside of not knowing you have a message while playing. To solve this I'm proposing adding a notification to the game when you receive a PM on the community. We could also hook other things (like topic reply notifications) as well if people were interested in that.
While it's not as clean as @mail I do think the proposed ideas fulfill the need left by its absence. Besides, there's not much else we can do.