Earlier this week Gmail's servers decided that any email sent from Gmail and then forwarded from pantz.org back to Gmail was now, as their servers put it "likely unsolicited mail". People sending mail from Gmail to pantz.org were getting bounce messages, which looks bad. All other email from any other domain was coming in without issue. I have been forwarding email from Gmail accounts for many years now without issue.
After seeing that everything checked out, I hit up Google to see if anyone else was having this issue. From the results it seems that many people have had this same issue. Some people just started using SRS to fix their issue. Others had to fix their PTR records in DNS. The last group of people had to stop using IPV6 for mail delivery. Since all of the other pantz.org mail server settings were correct, the only thing I could try was implementing SRS or turn off IPV6. Turning off IPV6 delivery was the easiest test. After turning off IPV6 mail delivery, and just leaving IPV4, all mail from Gmail being forwarded through pantz.org was now being accepted again. How dumb is that?
It seems Gmail has changed a setting (or I hit some new threshold) on their side dealing with only IPV6. Since Google will not tell you why certain mail is considered "unsolicited mail" we can not figure out what was done to try to fix the issue. If I had to speculate on what is happening, my guess is they turned up the sensitivity on email coming from IPV6 as it is obvious that IPV4 filter is not as sensitive. It is not just my server as it is happening to many other people as well.
I had also noticed that mail coming in from a friend whose server delivers mail to my server via IPV6, and then was forwarded to Gmail via IPV6 was being marked as spam every time. According to Google if the user is in your contacts list (and his email address is) the email is not supposed to be marked as spam. That is straight broken. Now that I switched back to just IPV4 delivery, all of his mail is not being marked as spam anymore. I believe Google has an issue with IPV6 mail delivery and spam classification.
I hate that I had to turn off IPV6 for mail forwarding to Gmail. My next likely step is to implement SRS for forwarding, and see if I can turn IPV6 back on. The best article I found on setting this up on Postfix is here. It also shows how to setup DKIM which might be fun to do as well.
This is quick writeup which describes connecting to a mail server and sending a test message using telnet.
Telnet to the mail server.
telnet mail.somedomain.com 25You should recieve a reply to this effect.
Connected to mail.somedomain.com.
Escape character is '^]'.
220 mail.somedomain.com ESMTP Exim 4.04 Wed, 13 Nov 2002 16:27:42 -0700
HELO mail.someotherdomain.comYou should get back something to the effect of this.
250 mail.somedomain.com Hello mail.someotherdomain.com [10.10.10.20]Now send your "From" command. This is the person the e-mail is from.
MAIL FROM:<Jim@someotherdomain.com>You should get the following back.
250 OKThen send your RCPT command. This is the person recieving the e-mail.
RCPT TO:<Joe@somedomain.com>You should get the following back.
250 OKThen send your DATA command. This is the body of your e-mail.
DATAYou should get back something from the server that looks like this.
354 Enter message, ending with "." on a line by itselfNow to create the body of the message. Put in the subject line and the message. Then put a period by itself as the last line and hit the "enter" key to finish.
This is a test message.
250 OK: queued as 18C6vd0005Ey-00Then type your last command to finish your session.
QUITYou will see the connection end
221 mailhub.somedomain.com closing connection Connection closed by foreign host