I maintain an application that sends out a ton of e-mail on a daily basis. Soon, we will have to migrate to another SMTP-server for that, which has an ip address that has no reputation with respect to email delivery. So instead of just flipping the switch, I would like to start by sending a small percentage of all mail through the new server, and then gradually increase that percentage until we reach 100%.

It wouldn't be very hard to implement something in the application itself, but I would like to know if there is an easier, more reliable out-of-the-box-type solution for this.

My first thought was to use round-robin DNS for this, but the servers require different credentials, use different protocols (one uses SASL, the other doesn't) and even different port numbers, so I think that rules out the DNS based solution.

Is there any way, for example, to configure Postfix to send 1 out of x e-mails to relay host A and the rest to relay host B? Or perhaps a different MTA?