OpenDKIM has not been updated in the last 7 years and failed to adopt
RFC8463, which introduces Ed25519-SHA256 signatures.
It has thereby held back the DKIM ecosystem, which relies on the DNS
system to publish its public keys. The DNS system in turn does not handle
large record sizes well (see RFC8301), which is why Ed25519 public keys
would be preferable, but I'm not sure the ecosystem has caught up, so we
stay on the conservative side with RSA for now.
Closes: #210
Add a certificate scheme for using an existing ACME certificate without
setting up Nginx.
Also use names instead of magic numbers for certificate schemes.
On the fly create certificates via openssl (Maybe change this to
libressl in the future?). This is probably the best scheme to get
something that simply works. Self signed certificates only pose a
problem when connecting to retrieve the email via imap or pop3.
