Proxied outgoing message transmission

In particular embodiments, a mailbox server is operable to receive, from a client, a message to be delivered to an endpoint; store the message in a mailbox associated with the user; access a set of policies for the client or the endpoint; determine a delivery channel for the message based on the policies; and transmit the message to the endpoint through the delivery channel. In particular embodiments, the mailbox server may transmit SMS messages via a special bind with a recipient's SMSC, or via a carrier short code. In particular embodiments, the mailbox server may transmit the message as an email or via an internal messaging system over a packet data connection.