store rejects for 14 days, and don't keep them as neutral by default so they won't cause outright rejects for repeated delivery attempts of spam messages

the previous default, marking the messages as junk had the interesting effect
of training the junk filter. rejecting could have been the result of the
sending IP being in the DNSBL. so the DNSBL helped to automatically train the
junk filter. perhaps we can keep that in the future and just not take messages
from the rejects mailbox into account when evaluating the reputation for
incoming deliveries.

smtpserver/server.go

@@ -2151,7 +2151,9 @@ func (c *conn) deliver(ctx context.Context, recvHdrFor func(string) string, msgW
 					log.Errorx("checking whether reject is already present", err)
 				} else if !present {
 					m.Seen = true // We don't want to draw attention.
-					m.Junk = true // This is junk, also train as such.
+					// Regular automatic junk flags configuration applies to these messages. The
+					// default is to treat these are neutral, so they won't cause outright rejections
+					// due to reputation for later delivery attempts.
 					m.MessageID = messageid
 					m.MessageHash = messagehash
 					acc.WithWLock(func() {