OpenDKIM
C implementation of DKIM (RFC 6376), forked from Sendmail's legacy dkim-milter at IETF ratification of the standard. The project is effectively feature-complete: development is quiet because the protocol is over twenty years old and stable, not because the software is neglected.
Suitable for any deployment scale. As a native C milter it operates
with near-zero overhead — processing times well under a tenth of a
second make throughput a non-issue in practice. Multi-domain
configurations are handled natively via keytable and signingtable,
with no external tooling required. Key rotation is standard
administration, not a scalability concern.
The canonical limitation is shared by any milter architecture: the MTA must transmit the message to the filter process, which introduces a round-trip. In a monolithic design this overhead disappears, but at sub-100ms processing times the difference is operationally irrelevant.
Tested in production on Sendmail 8.x for over a decade. No known stability issues.
Note: OpenDKIM implements DKIM1 (RFC 6376). For DKIM2 support see DarkChain.