> Another possible test: > > #ifconfig lo0 mtu 1500 > > That should decide any argument about write buffer sizes.
Was about to start with tcpdump, but changing the mtu resolves the issue entirely, confirming your earlier thoughts. There is now no significant delay for mail submitted with large attachments and throughput for such mail is similar to the results I obtained using smtp-source. My sincerest of thanks for providing both a solution and information about OpenBSD semantics of which I was not aware. Kind regards, Dave Green
