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 >