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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.

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Comcast DMARC Update
To help us improve our detection of those who use the domain maliciously we have published a DMARC record for, but that change WILL NOT disrupt legitimate messaging.

More information our DMARC policy updates here.
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
New limit on recipients
We've changed our policy on how many recipients can be addressed in a single email.  Check out for details.