This has happened to me a couple of times over the past ~2 years. I'm on 
a *nix based mail system operated by campus IT, not gmail.

Bob

On 9/15/2016 9:16 AM, Eric Hellman wrote:
> Have others been having difficulty staying subscribed? It seems gmail doesn't like the CLIR listserv.
>
>> - The first error was reported on 2016-09-09.
>>
>> - Since then, a total of 10 delivery errors have been received.
>>
>> - The last reported error was: 5.7.1 550-5.7.1
>> [2600:3c03::f03c:91ff:fe55:a961]  Our  system   has  detected  that  this
>> 550-5.7.1 message  does not  meet IPv6  sending guidelines  regarding PTR
>> records   550-5.7.1   and   authentication.   Please   review   550-5.7.1
>> https://support.google.com/mail/?p=IPv6AuthError <https://support.google.com/mail/?p=IPv6AuthError> for more information 550
>> 5.7.1 . g16... (23 bytes suppressed)
>
>
> Eric Hellman
> President, Free Ebook Foundation
> Founder, Unglue.it <http://unglue.it/> https://unglue.it/ <https://unglue.it/>
> https://go-to-hellman.blogspot.com/ <https://go-to-hellman.blogspot.com/>
> twitter: @gluejar
>