My guess is this has something to do with DKIM.
I don't like DKIM because it doesn't always work with forwarded mail,
even when the forwarding is RFC-compliant.
*sigh*

(Typically these alumni addresses--I have one at my alma mater--receive
email and then forward it somewhere else.)

Nelson Pardee <[log in to unmask]> wrote:
> What's going on it very odd, and I'm seeing some possibly related
> problems
> in another context.
> The person experiencing this problem is our new list co-owner, a
> completely
> legitimate person. She just got an email from her alma mater as an
> alumnus;
> these emails are setup and managed by google in the alumni.usc.edu
> domain.
> She recently received, for example, an email from someone having a
> gmail
> address with the message "Gmail could not verify that it actually came
> from
> [log in to unmask]". In fact, the sender has sent her dozens of emails-
> there is
> nothing fake or phony about the email. I infer that the spam filters on
> the
> alumni.usc.edu domain are overly aggressive and/or wrongly configured