<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Dec 21, 2017, at 5:26 PM, Michael Stauber <<a href="mailto:mstauber@blueonyx.it" class="">mstauber@blueonyx.it</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">That's my experience as well: If CentOS 7 is hooked up to a network that</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">supports IPv6 (even if that server only has IPv4 configured), then some</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">services might end up using IPv6 as well. At least for outgoing</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">connections if the target supports IPv6. In that case it might even</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">prefer to use IPv6 even if IPv4 is available at the destination.</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">There are some factors that can throw a wrench into that. Like if you</span><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">use a DNS-resolver that only serves IPv4.</span></div></blockquote></div><br class=""><div class="">This is really starting to chap by backside. On my other 5208R (CentOS 6.9) that I’ve not had this issue with - Just had a customer notify me that they were having trouble sending to Comcast with this error:</div><div class=""><br class=""></div><div class=""><blockquote type="cite" class="">From: Mail Delivery Subsystem [<a href="mailto:MAILER-DAEMON@box1.qzoneinc.net" class="">mailto:MAILER-DAEMON@box1.qzoneinc.net</a>] <br class="">Sent: Friday, December 22, 2017 3:03 PM<br class="">To: <a href="mailto:agent@davisjohnstone.com" class="">agent@davisjohnstone.com</a><br class="">Subject: Returned mail: see transcript for details<br class=""><br class="">The original message was received at Fri, 22 Dec 2017 15:02:56 -0600 from<br class=""><a href="http://cpe-24-243-103-228.rgv.res.rr.com" class="">cpe-24-243-103-228.rgv.res.rr.com</a> [24.243.103.228]<br class=""><br class=""> ----- The following addresses had permanent fatal errors -----<br class=""><<a href="mailto:susanmyerscough@comcast.net" class="">susanmyerscough@comcast.net</a>><br class=""> (reason: 554 <a href="http://resimta-po-03v.sys.comcast.net" class="">resimta-po-03v.sys.comcast.net</a><br class=""><a href="http://resimta-po-03v.sys.comcast.net" class="">resimta-po-03v.sys.comcast.net</a> 2605:6000:7b44:b00:21d:9ff:fe1e:9f...<br class="">requirement. For more information, refer to:<br class=""><a href="http://postmaster.comcast.net/smtp-error-codes.php#554" class="">http://postmaster.comcast.net/smtp-error-codes.php#554</a>)<br class=""><br class=""> ----- Transcript of session follows ----- ... while talking to<br class=""><a href="http://mx2.comcast.net" class="">mx2.comcast.net</a>.:<br class=""><<< 554 <a href="http://resimta-ch2-24v.sys.comcast.net" class="">resimta-ch2-24v.sys.comcast.net</a> <a href="http://resimta-ch2-24v.sys.comcast.net" class="">resimta-ch2-24v.sys.comcast.net</a><br class="">2605:6000:7b44:b00:21d:9ff:fe1e:9fe Comcast requires that all mail servers<br class="">must have a PTR record with a valid Reverse DNS entry. Currently your mail<br class="">server does not fill that requirement. For more information, refer to:<br class=""><a href="http://postmaster.comcast.net/smtp-error-codes.php#554" class="">http://postmaster.comcast.net/smtp-error-codes.php#554</a><br class="">... while talking to <a href="http://mx1.comcast.net" class="">mx1.comcast.net</a>.:<br class=""><<< 554 <a href="http://resimta-po-03v.sys.comcast.net" class="">resimta-po-03v.sys.comcast.net</a> <a href="http://resimta-po-03v.sys.comcast.net" class="">resimta-po-03v.sys.comcast.net</a><br class="">2605:6000:7b44:b00:21d:9ff:fe1e:9fe Comcast requires that all mail servers<br class="">must have a PTR record with a valid Reverse DNS entry. Currently your mail<br class="">server does not fill that requirement. For more information, refer to:<br class=""><a href="http://postmaster.comcast.net/smtp-error-codes.php#554" class="">http://postmaster.comcast.net/smtp-error-codes.php#554</a><br class="">554 5.0.0 Service unavailable<br class="">Reporting-MTA: dns; <a href="http://box1.qzoneinc.net" class="">box1.qzoneinc.net</a><br class="">Received-From-MTA: DNS; <a href="http://cpe-24-243-103-228.rgv.res.rr.com" class="">cpe-24-243-103-228.rgv.res.rr.com</a><br class="">Arrival-Date: Fri, 22 Dec 2017 15:02:56 -0600<br class=""><br class="">Final-Recipient: RFC822; <a href="mailto:susanmyerscough@comcast.net" class="">susanmyerscough@comcast.net</a><br class="">Action: failed<br class="">Status: 5.5.0<br class="">Diagnostic-Code: SMTP; 554 <a href="http://resimta-po-03v.sys.comcast.net" class="">resimta-po-03v.sys.comcast.net</a> <a href="http://resimta-po-03v.sys.comcast.net" class="">resimta-po-03v.sys.comcast.net</a> 2605:6000:7b44:b00:21d:9ff:fe1e:9fe Comcast requires that all mail servers must have a PTR record with a valid Reverse DNS entry. Currently your mail server does not fill that requirement. For more information, refer to: <a href="http://postmaster.comcast.net/smtp-error-codes.php#554" class="">http://postmaster.comcast.net/smtp-error-codes.php#554</a><br class="">Last-Attempt-Date: Fri, 22 Dec 2017 15:02:58 -0600</blockquote><br class=""></div><div class="">Looked to me like Comcast was picking up the IP of the desktop sending the message to my server (on <a href="http://rev.res.rr.com" class="">rev.res.rr.com</a>), so I enabled ‘hide previous headers’ and had my customer try again. Failure with the same message. Acting on a hunch that they were blocking my non-PTR IPv6 (and not telling me), I applied the faux IPv4/IPv6 CLIENT_OPTIONS line to this server and the mail was accepted.</div><div class=""><br class=""></div><div class="">I swear… Running a well configured mail server has never been easy, but these arbitrary requirements of PTR records for mail delivery on a not quite ready for prime time IPv6 are making this a nightmare.</div><div class=""><br class=""></div><div class="">I hope everyone has a blessed and merry Christmas!</div><div class=""><br class=""></div><div class="">Cheers,</div><div class="">Jeff</div><div class=""><br class=""></div></body></html>