The process is actually very similar to text/email. If you select your contact via messenger and choose their email address, messenger will handle it like a text and the recipient will show your #@cell provider as the sender. If you are sending a 'messenger email' (text or text + documents) using Messenger to a FB contact's Messenger, the easiest way would be to use Messenger for all functions. Messenger is however you choose to use it, e.g. texting, image sharing, document sharing. There are some file limitations but overall it can manage a decent amount of tasks.