ensure senderaccount is always set for messages in queue

before, the smtpserver that queued a dsn would set an empty senderaccount,
which was interpreted in a few places as the globally configured postmaster
cacount. the empty senderaccount would be used by the smtpserver that queued a
dsn with null return path. we now set the postmaster account when we add a
message to the queue. more code in the queue pretty much needs a non-empty
senderaccount, such as the filters when listing, and the suppression list.

queue/hook.go

@@ -204,11 +204,9 @@ func cleanupHookRetiredSingle(log mlog.Log) {
 
 func hookRetiredKeep(account string) time.Duration {
 	keep := 24 * 7 * time.Hour
-	if account != "" {
-		accConf, ok := mox.Conf.Account(account)
-		if ok {
-			keep = accConf.KeepRetiredWebhookPeriod
-		}
+	accConf, ok := mox.Conf.Account(account)
+	if ok {
+		keep = accConf.KeepRetiredWebhookPeriod
 	}
 	return keep
 }