+This accepts a client certificate that is verifiable against any
+of your configured trust-anchors
+which usually means the full set of public CAs)
+and which has a SAN with a good account name.
+Note that the client cert is on the wire in-clear, including the SAN,
+whereas a plaintext SMTP AUTH done inside TLS is not.
+
+. An alternative might use
+. .code
+. server_param1 = ${sha256:$tls_in_peercert}
+. .endd
+. to require one of a set of specific certs that define a given account
+. (the verification is still required, but mostly irrelevant).
+. This would help for per-device use.
+.
+. However, for the future we really need support for checking a
+. user cert in LDAP - which probably wants a base-64 DER.
+