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Postmaster FAQs

This page was designed to address questions on the following topics:

All policies and thresholds on this page are subject to change. This site will be updated when such changes are implemented.

WhatâNew @ Postmaster  
Comcast Implements DMARC Validation
For the past two years, Comcast has contributed to the Domain-based Message Authentication, Reporting and Conformance specification (DMARC). We are pleased to announce that we have now implemented DMARC for to provide additional protection against spam and phishing messages.

Sending domains can publish a DMARC policy in DNS. By doing this the domain can specify what action Comcast should take if they receive unauthenticated messages (no action, quarantine, or reject). Initially, Comcast will treat a "reject" policy request by a sending domain the same as "quarantine" and so will mark unauthenticated messages in both categories as spam.

Comcast has also implemented DMARC aggregate reporting to provide feedback to sending domains about mail claiming to be from their domain and what DMARC actions Comcast took. Reports will be sent daily from

For more information on DMARC, visit
Sending to Comcast over IPv6
As part of World IPv6 Day on June 6th, Comcast will begin accepting mail from senders with IPv6 addresses. Information on our policies and how to avoid blocks when sending can be found here.
Exchange servers and Comcast DNSSEC
With the recent launch of the Comcast DNSSEC platform, and with other ISPs soon to follow, some Exchange server admins may notice intermittant delivery issues. These appear to be related how they determine DNS resolution and from which source.
We've obtained some information from Microsoft and their website that may help resolve the issue and have posted it to our help forums here.
SMTP Error Codes Information Updated
We've revised our information concerning the various SMTP error codes senders and users may see when communicating with message systems.
These codes are documented at
Certified Email is being discontinued
At Comcast, we take online security very seriously. We partner with industry-leading companies that provide cutting-edge security technology to ensure you have the best online experience. Whether it's spam reduction or helping to protect against viruses, phishing, identity theft and other online threats, we never stop strengthening our network protection.

We are writing to inform you that one of our partners, Goodmail, has ceased operations and will no longer be integrated within Comcast's services. However, we want to assure you that we will continue to provide the most comprehensive suite of online security and protection services available from any Internet service provider.

XFINITY Internet customers can visit to get the latest security software updates, news, tips and information to help keep you and your family safe online. Customers will also find information on how to download Norton Security Suite, activate Secure Backup & Share and other great services, all included at no additional charge with XFINITY Internet service.

What this means to you:
If you check your email through the SmartZone Communications Center on or, you may have noticed a Goodmail envelope icon next to emails from certain retailers, government agencies and financial institutions. This icon will stop being displayed as of 2/7/11. As always, we encourage customers to use email responsibly and use judgment when opening emails.and clicking on links within those emails.if they come from questionable sources.
New limit on recipients
We've changed our policy on how many recipients can be addressed in a single email.  Check out for details.