Hacker's ramblings

I covered Google's new & ridiculous e-mail policy in my previous post.

The author of my favorite MTA, Postfix, Mr. Wietse Venema offered a piece of advice to another poor postmaster like me in the official Postfix User's Mailing list "disable ipv6 when sending to gmail?"

The idea is to use Postfix's SMTP reply-filter feature. With that, postmaster can re-write something the remote server said into something useful to alter Postfix's behavior. In this case, I'd prefer a retry using IPv4 instead of IPv6. Luckily the ability of dropping down to IPv4 is already built in, the only issue is to convince Postfix that what Google said is not true. For the IPv6-issue they state that the e-mail in question cannot be delivered due to a permanent error. A status code of 5.5.0 is given in this case. What Wietse suggest is to re-write the 5.5.0 into a 4.5.0 which indicates a temporary failure. This triggers the mechanism to do an IPv4 attempt immediately after failure.

I added following into /etc/postfix/main.cf:
# Gmail IPv6 retry:
smtp_reply_filter = pcre:/etc/postfix/smtp_reply_filter

Then I created the file of /etc/postfix/smtp_reply_filter and made it contain:
# Convert Google Mail IPv6 complaint permanent error into a temporary error.
# This way Postfix will attempt to deliver this e-mail using another MX
# (via IPv4).
/^5(\d\d )5(.*information. \S+ - gsmtp.*)/ 4${1}4$2

Reload Postfix just to make sure the main.cf change is in effect, no need to postmap the PCRE-file.

Effectively the last line of Google error message:

550-5.7.1 [2001:-my-IPv6-address-here-      16] Our system has detected
550-5.7.1 that this message does not meet IPv6 sending guidelines regarding PTR
550-5.7.1 records and authentication. Please review
550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_error for more
550 5.7.1 information. dj7si12191118bkc.191 - gsmtp (in reply to end of DATA command))

will be transformed into:

450 4.7.1 information. dj7si12191118bkc.191 - gsmtp (in reply to end of DATA command))

And my mail gets delivered! Nice. Thanks Wietse! Shame on you Google!