+ it allows other messages through.
+
+ We also wait for the next retry time if this is a message sent down an
+ existing SMTP connection (even though that will be forced). Otherwise there
+ will be far too many attempts for an address that gets a 4xx error. In
+ fact, after such an error, we should not get here because, the host should
+ not be remembered as one this message needs. However, there was a bug that
+ used to cause this to happen, so it is best to be on the safe side. */