On Tue, 2011-06-14 at 00:24 -0400, Phil Pennock wrote: > You're using sender *callout* verification to systems not under your
> administrative control.
That looks suspiciously close to identical to the OP's problem, doesn't
it? So, allowing for that, let's say that there was possibly a transient
DNS error which meant that the MTA in quuestion could not look up the
sender domain.
Religion, pfft. Let's deal with it scientifically, please.