Synchronized Forwarding in Ibex Mail
Messages to be forwarded to another server are never enqueued locally; instead, Ibex Mail acts as an intelligent proxy between the inbound SMTP connection and a new connection to the remote server, rewriting message headers “on the fly”. An earlier reference to this idea appears here.
Because Ibex Mail never assumes responsibility for delivering these messages it never needs to generate bounce messages for nonlocal users. This means that an Ibex Mail server can never be a source of any amount of backscatter; bounce messages are only sent to users who use the Ibex Mail server as their own outbound MTA.
Here is an example, where user email@example.com is sending mail to firstname.lastname@example.org (running Ibex Mail), who has forwarded his mail to email@example.com:
The only (minor) drawback of this approach is that the sending MTA, the forwarding MTA (Ibex Mail), and the receiving MTA must all be reachable simultaneously in order for delivery to occur. With a scheme in which the forwarding machine accepts responsibility for a message it is possible for delivery to occur in a scenario where the sending and receiving MTAs are never online simultaneously (but each is occasionally online at the same time as the forwarding MTA).
In today's modern Internet, situations such as the above are vanishingly rare, and almost always symptomatic of a malfunction which ought to be fixed.