This affected any platform not supporting "abstract" Unix-domain
sockets (i.e. not Linux).
+JH/42 Bug 2693: Harden against a peer which reneges on a 452 "too many
+ recipients" response to RCPT in a later response, with a 250. The
+ previous coding assumed this would not happen, and under PIPELINING
+ would result in both lost and duplicate recipients for a message.
+