Treat a QUIT error as merely a warning: RFC 2821 only mandates that
a QUIT error should abort an unfinished transaction, and since we've
reached this point, the DATA command has succeeded and the message has
been accepted for delivery by the remote end.  Thus, just warn about it.

--- a/net.c
+++ b/net.c
@@ -462,11 +462,9 @@
 	}
 
 	send_remote_command(fd, "QUIT");
-	if (read_remote(fd, 0, NULL) != 2) {
-		syslog(LOG_ERR, "%s: remote delivery deferred: "
+	if (read_remote(fd, 0, NULL) != 2)
+		syslog(LOG_WARNING, "%s: remote delivery succeeded but "
 		       "QUIT failed: %s", it->queueid, neterr);
-		return (1);
-	}
 out:
 
 	close(fd);