I want to setup Domain Key signatures for outbound mailers (postfix). I am currently using dkimproxy but I would need to setup one instance per postfix which is too many. Is there software that will sign for all the mail servers on one box?
What are the other options out there?
Take a look at dkim-milter. MTA's connect via a socket and it can handle signing with differing keys. In testing a reasonable (4x Opteron, 4G RAM, 1-7200 RPM Sata drive) machine was able to handle accepting, signing, and deliver of 5k messages in under a minute.
The MTA must be able to use milter type filters. Postfix has been able to do so since about 2.3 with various levels of support.
We use dkim-milter with postfix as well and does what you want. The documentation is a bit skimpy if I recall correctly, and our guy used this howto.