Native SRS is a nice thing to have, especially if you are forwarding to Gmail.

A shame that DKIM wasn’t built in too, but there’s a filter/plug-in architecture and you can add a third party filter to do DKIM for you.

Sometimes a commitment to supporting a plug-in architecture can be a little overzealous. Yes, it’s a nice general purpose puzzle to solve if you are the engineer writing the code. When DKIM is such a critical part of email in 2022, I wouldn’t mind if it were treated as a special case and supported by default.