DKIM Signatures are created missing important headers (RFC 6376 §5.4 recommends Cc, Content-Type, MIME-Version, Content-Transfer-Encoding, …) because it is hardcoded in the impl Default for Dkim1Signature.
This can be update but on automatic key rotation those changes are lost.
It will be nice if we had two new features:
- make the default list configurable on per domain or the entire server
- copy headers from the key being rotated to make the server continue behaving as it was before key rotation
For now, users wanting a better default can compile its own version including more headers changing the hardcoded list at crates/registry/src/schema/structs_impl.rs:7333.