+PH/17 Added dsn_from option to vary the From: line in DSNs.
+
+PH/18 Flush SMTP output before performing a callout, unless control =
+ no_callout_flush is set.
+
+PH/19 Change 4.64/PH/36 introduced a bug: when address_retry_include_sender
+ was true (the default) a successful delivery failed to delete the retry
+ item, thus causing premature timeout of the address. The bug is now
+ fixed.
+
+PH/20 Added hosts_avoid_pipelining to the smtp transport.
+