Additional information: It seems to have something to do with the
authentication method used by the listserv. There's an article here about
it, that they suggest be passed on to the managers of a list from whence
you are receiving these messages:
https://support.google.com/mail/answer/180707?hl=en

Marc P.

On Sat, Apr 26, 2014 at 10:54 PM, Marc Perdue <mcperdue@gmail.com> wrote:

> I don't know the actual answer, but it appears that Google has made some
> modifications to its spam filters, probably in response to the recent
> Heartbleed issues. At first, I was getting these mostly in response to
> listmembers who had Yahoo and Aol accounts. It's ironic that the filters
> are suspicious of Google's own gmail accounts.
>
> Marc P.
>
>
> On Sat, Apr 26, 2014 at 10:47 PM, bernie <1234bjs@gmail.com> wrote:
>
>> Yes I have and I just got one.
>>
>> "This message may not have been sent by: dhanson928@gmail.com "
>>
>> I was wondering the same thing. Why now all of a sudden?
>>
>> Bernie
>>
>>
>> On Sat, Apr 26, 2014 at 7:44 PM, Don Hanson <dhanson928@gmail.com> wrote:
>>
>> >   Has anybody else recently began getting these Gmail  "!Warning! May
>> not
>> > have been sent by ____________......" message headers with a lot of
>> vanagon
>> > list  emails?   They began appearing a few days ago,  on top of lots of
>> the
>> > posts from Vanagon List that I've been getting.  Gmail has a convoluted
>> > explanation in a drop down menu that I don't follow, being a
>> > cyber-challenged...I  don't think I have changed anything in my
>> settings to
>> > cause these highlighted headers to began appearing...but ....?
>> >
>> >   Don Hanson
>> >
>>
>
>