Email, like in many multiplayer games is a means of communicating with other players, and more importantly, sending items to other players.
Items can be attached, but there is a loophole to be aware of, that of refurbishment.
"Tradeable" is a flag that exists on an item which can read "Yes" or "No". Many times if the item is not tradeable, it must first be "Refurbished".
To refurbish an item, you simply access your Locker (L), browse to the category and the item in question, and click "Edit this Item".
You will have several options, one of which is duplicate, and another, refurbish.
As an example, you create a SWAT Van and want to send one to a friend. Choose the Van from your Locker, edit it. Choose duplicate. Choose refurbish. Now you have a duplicated refurbished and tradeable van in your locker.
Access the email terminal, compose an email to your friend, click the attachment button. Add the van. Fill out a subject whatever you'd like to say in the email, and send.
Your friend will receive the email, and upon accessing the attachment, add the vehicle to his locker, where he can then equip it from a joker ammunition terminal by pressing I, and clicking the vehicle tab.