Dave Green:
> > 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.
It's no different with Linux, Solaris, and so on.
The loopback MTU is usually large to improve performance, but that
happens only with programs that take advantage of it,
I suggest that you inform the developers of the mail sending
application so that they can fix the problem on their end.
Changing the lo0 MTU is not a permanent solution.
Wietse
