for localserve, validate the incoming message

it must be completely parsable.
normally, if we receive a message that we cannot fully parse, we accept it and
treat it as opaque application/octet-stream.

also make it more clear that localserve accepts email intended for any email
address.

doc.go

@@ -307,8 +307,8 @@ automatically initialized with configuration files, an account with email
 address mox@localhost and password moxmoxmox, and a newly generated self-signed
 TLS certificate.
 
-All incoming email is accepted (if checks pass), unless the recipient localpart
-ends with:
+All incoming email to any address is accepted (if checks pass), unless the
+recipient localpart ends with:
 
 - "temperror": fail with a temporary error code
 - "permerror": fail with a permanent error code