I wish to host some managed email servers for some customers. Each customer will have their own email server which will be an all-in-one virtual machine running postfix, dovecot and some webmail suite.
Even though each customer will have their own server, I do not wish to give each email server it's own public facing IP. I wish to avail the use of proxy servers so all customers use the same public IP. As for the "smtp-in" from the public internet, this isn't a problem as I can set up many mx servers (using postfix) which will store-and-forward the mail to the correct server (using transport maps). As for the IMAP access from the customer, I was thinking of using perdition which is an IMAP proxy - I believe that this will suit my needs.
I am confused however on what to use for the "smtp-out" proxy. The customers will have to authenticate with their receptive email server, however they will have to go via a proxy of some sort as they won't have direct access to their server instance. It probably can't be a store-and-forward proxy either.
Does anyone have any idea on what I could use here?