SRS Header Rewrites Broken in Many E-Mail Servers - Causing "550" Errors

While the content of this message is specifically applicable to the SmarterTools, SmarterMail e-mail server product, it is of extreme importance to everyone who uses or hosts their own e-mail.

It turns out that, since sometime around SmarterMail version 10.X or 11.X, the ability of SmarterMail to properly do something called SENDER HEADER REWRITE has not been functioning properly and this is causing 550 NON DELIVERABLE ERRORS on e-mail which is FORWARDED from someone's SmarterMail account.

 

For those of us who are involved, on a day-to-day basis with e-mail services, and troubleshooting those services, this has been a thorn in our sides since OUTLOOK.COM, YAHOO!, GMAIL, COMCAST, and many other e-mail providers, ChicagoNetTech included, have enforced DMARC and other antispam protocols in an effort to keep our user's in boxes as fee of spam as possible.


 

For those who have been sending e-mail and seeing 550 X.X.X errors which specify that their e-mail was not delivered because of an SMTP AUTHENTICATION ERROR, along with the techs who have been attempting to troubleshoot these non-delivery issues, this has been an absolute nightmare, to all of whom who have so suffered, I sincerely apologize.

Some background:



 

In our efforts to prevent spam from being forwarded via an unauthorized source, many ISPs are now performing hard checks SMTP + SPF | Protected by SPFon all incoming messages to ensure that the SPF included in the message header actually matches the SPF information for the domain which is forwarding the message.


 

When someone has their e-mail setup to FORWARD to another account, unless SRS has been enabled by the relaying MX server, the SPF no longer matches the original sender's MX to SPF mappings and the messages are now frequently rejected by the receiving MX server.  This is now being enforced by Gmail, YAHOO! and COMCAST, as well as ChicagoNetTech, as we look for additional ways to prevent spammers from "gaming the system" and they continue to find new ways to work-around and circumvent the latest spam prevention techniques.

While this is almost always an inconvenient incumbrance to the party who's forwarded e-mail is rejected, after being forwarded, most modern MX servers have a way to insert updated header information into the message, leaving the original headers intact, and adding routing information to the message header so those MX servers receiving forwarded messages will not reject the forwarded message.

In SmarterMail, this is easily enabled by the primary MX server administrator by ENABLING SRS to REWRITE the MESSAGE HEADER as it passes through the SmarterMail MX server.

 

Is There Anything Else to the Equation?

In February, 2013, a new player was introduced to the antispam field.  Called DMARC, this tool is designed to allow e-mail server operators to tell those e-mail servers who respect the DMARC protocol what to do with messages which originate from sources other than authorized e-mail servers.  The immediate effect of the DMARC protocol was that it almost completely eliminated job-jobbing, a form of e-mail spoofing which usually caused major headaches for the sender who's e-mail address had been hijacked as the "reply to" address on bogus e-mail messages.

DMARC checks for valid:

 

      DomainKeys: a private certificate, generated specifically for a domain, and mapped to the domain both in the messages header and in the domain's DNS, which validates the ability of that domain to originate an e-mail message;

      DKIM Records: digital signing of specific fields of a message, tied to the DomainKey of the domain from which the message was sent;

      SPF Records: Sender Policy Framework records were one of the original tools adopted to ensure the authenticity of messages and a very early form of authentication which tied IP addresses and domains together;

      rDNS or IN-ARPA Records: a reverse address mapping of the IP Address from which the message was sent to the Fully Qualified Domain Name (FQDN) of the e-mail server with the authority to originate a message on behalf of a domain. 

 

 

 

Because DMARC is a flexible protocol, designed to allow e-mail server operators the capability to gradually "turn up the heat" on spam messages received at receiving e-mail servers, an issue with message forwarding was not readily apparent, until about a year ago, when GMAIL turned up the head and started to completely restrict messages which did not absolutely meet the DMARC protocol.

So, How is SRS Rewrite Supposed to Work? 

With that in mind, let's go back and look at how SRS Header Rewrites are supposed to work.

In traditional, or verbatim, forwarding, the return-path is preserved in the outgoing message.  Here's a graphic to help illustrate the issue:

Forward WITHOUT SRS Forwarding: Header Rewrite - will fail

and here's a graphic with the SRS rewrite added - allowing the message to pass SPF when received at the new mailbox 

Forward WITH SRS Forwarding: Header Rewrite - will PASS

In an SPF world, forwarders need to rewrite the return-path to stay in good standing.  

The SRS library implementations help perform the needed transformation and ensure proper delivery.  Full-time forwarding services may prefer to integrate this logic directly into their MTAs.  Smaller sites don't have to: they can just call the srs utility which does all the work.

Once you have enabled SRS, additional header information will be written into the header of the original message, allowing it to meet the SPF compliance for the SmarterMail server which is forwarding, or RE-SENDING the message to the party's off-site e-mail address, without the receiving MX server rejecting the message for an SPF error. 

When the full force of DMARC is applied, and an SPF error occurs because of a failed SRS header rewrite scenario, we end up with what is referred to as a 550 ERROR

But wait, the 550 ERROR message says that the message was rejected because the sender did not use SMTP AUTHENTICATION, and that's something that's commonly checked as part of the antispam measures, to prevent spam from reaching all of our in boxes, right?

Yes, but the 550 ERROR is also the error which is returned with there is a MISMATCH on the SENDING E-MAIL ADDRESS DOMAIN because it is not included in the SPF RECORD!

 

Pictures are Sometimes Very Helpful!

Here's a picture which might help you understand the issue better:

 

Point of information:  an MTA is another way to reference an e-mail server.

So, the ORIGINAL MTA sends the message to the RECEIVING MTA. 

The receiving MTA then validates all of the informaiton in the message header against a REMOTE MTA, generally a DNS SERVER, which contains information on what server is responsible for what kinds of services for a specific domain name and its services, and then reports back to the REPORTING MTA whether the information contained in the message headers is GOOD or BAD.

When the SRS header is not properly rewriteen, the REMOTE MTA reports back to the REPORTING MTA as BAD DATA and the REPORTING MTA rejects the message, via the RECIVING MTA with a 550 ERROR.

 

 

 

 

 

The receiving e-mail server is rejecting a legitimately forwarded e-mail message because it thinks it is from a spammer, IE: the SENDING DOMAIN is not authorized, under SPF, to send on behalf of the domain which actually sent the message

It turns out the 550 ERRORS are NOT a spam issue, but a FORWARDING ISSUE -- a FORWARDING ISSUE which is caused by the fact that SRS HEADER REWRITES were not properly written into the message header (a part of every e-mail message which tells the receiving e-mail server where it came from, who sent it, how the recipient should reply, etc, and also contains important routing and antispam information.

 

Summary:

The messages are being rejected because SmarterMail, along with many other MX servers, is not properly doing a SRS FORWARD HEADER REWRITE and the messages appear to be SPAM which was sent through an OPEN RELAY.

I've opened a ticket with our SmarterMail vendor, SmarterTools.  You can read the ticket, comments, and progress, at: https://portal.smartertools.com/community/a2676/srs-header-rewrite-not-working-properly-on-message-forwarding-_-_-_.aspx

You can also see my original KB (Knowledge Base) article on SRS Forwarding at: https://portal.chicagonettech.com/kb/a177/enable-srs-to-prevent-spf-fail-on-forwarded-e-mail-messages.aspx

While this particular instance relates to a software problem with a specific product, SmarterMail, this is not an uncommon situation with e-mail servers, which are also known as MX servers and MTAs.

If you have experienced issues with 550 errors and e-mail rejection, you may want to forward this e-mail to your IT Department or e-mail support group so they can look into whether their e-mail server product is properly inserting SRS Header Rewrites into e-mail message headers.

Remember, ChicagoNetTech provides secure e-mail and web hosting services and will be glad to help you make your online experience better.

Bruce Barnes ஃ

ChicagoNetTech Inc

SmarterTools Product Specialist
E-Mail and DNS Security Specialist
Network Security Specialist

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