It would be great if Mailgun API is extended to allow modifying the message on the server, before sending it out, without downloading.

Our use case is very simple. We want to update FROM, TO, CC, BCC for each received message and send it out. The current API allows this use case, but requires that each message is entirely downloaded by our services, including all possibly big attachments, and uploaded back to Mailgun after updating. This is a lot of wasted network bandwidth both ways and highly increases processing / delivery times of each message.

If Mailgun API is extended to allow updating the message on the server side, before it's sent out, that would highly streamline the processing, saving time, money & network bandwidth.