New Code!
Postoffice has been pushed up to version 1.5.0 with the addition of the new configuration option mxpool, which better supports domains with multiple mail exchangers.
The traditional behavior of postoffice is to deliver mail locally if the current machine is an mx for the destination domain. This is all fine and good if you’ve only got one mail server, but if you’re running secondary mail servers to grab the mail when the primary dies (an important configuration these days now that most of the big commerical network providers have ridiculously short delivery timeouts) it doesn’t work very well (I’ve tried setting up virtual domains and relaying that way, but that’s not a particularly clean way of doing things.) 1.5.0 and mxpool changes that behavior; if mxpool is enabled, mail will not be delivered locally unless the local machine is the highest priority mail exchanger for the offending domain; if the machine is a lower priority mail exchanger, it will accept the mail as if it was local, but it will then forward that mail off to the highest priority mail exchanger it can reach.
This is New Code!, and has not been heavily tested; I am running it on several domains with paired primary/secondary mail exchangers, and the secondary exchangers appear to be forwarding the mail off to the primary without any fuss or muss or bother, but I’m just one system administrator and, worse yet, I’m the developer of this thing. For other people the traditional New Code! caveats apply, up to and including the recommendations that you test it only in a sealed decontamination chamber, but if it doesn’t spontaneously cause mutations it’s a useful feature for a mail server to have.