This is now possible with a custom header that will force JangoSMTP to assign an email to a particular account:
If you include this header in each email message, then the message will be assigned to the JangoSMTP account with the username designated in the header, regardless of the IP address from which it is sent. The sending IP address would still need to be added to one of the JangoSMTP accounts, but it wouldn't matter which account.
For example, let's say you're a web development agency, and you have five clients, and you host all five of your clients' web sites on a shared server with a single IP address. You might create five JangoSMTP accounts with the usernames:
to assign that email to the PizzaFactory account, for example.