Comcast deliverability best practices

Sending successfully to Comcast and reaching the inbox is highly dependent on your sending reputation and following basic sending best practices. While following best practices doesn’t guarantee inbox placement at Comcast, it greatly increases your chances for success.

One of the best ways to reach the inbox at Comcast is sending relevant content that your subscribers want to receive. Low complaint rates and having subscribers marking your email as “not spam” signals to Comcast that you are less likely a spammer.

Best Practices

• Enroll all sending IP addresses in the Comcast complaint feedback loop and add all complainers to a suppression list.
  
  • Comcast auto-approves requests. Requests that are not approved automatically sit in their queue as pending, but are eventually denied.
  • Full circle DNS (FCrDNS) for your IPs is required.
• Ensure all sending IP addresses have a non-generic, unique reverse DNS (PTR) record. Connections to Comcast servers will not be accepted from an IP address without a PTR record.
• All sending domains must have a valid MX or A record. Connections to Comcast servers will not be accepted from a sending domain without an MX or A record.
• Use Sender Policy Framework (SPF), Domainkeys Identified Mail (DKIM), and Domain-based Authentication, Reporting, and Conformance (DMARC) for all email.
• If you publish a DMARC record with a reject policy, ensure SPF is updated with all sending IP addresses and that there are no errors with DKIM. Comcast will only block your email with a DKIM and SPF authentication failure along with a DMARC reject policy.
• Remove all unknown users after one bounce. Comcast considers an unknown user bounce-back to a sender as an unsubscribe request.
• Comcast deletes user email accounts after 12 months of inactivity. Ensure your win-back and suppression policies are set appropriately. Senders with longer selling or seasonal sending cycles may need to be more aggressive with win-back campaigns or with attempts to request updated information from inactive Comcast subscribers.
• Don’t send to an old list or suppression list that have a large number of unknown users. Comcast will block your IP address if they detect a large number of unknown users.
• Use opt-in or confirmed opt-in permission methods when acquiring subscribers along with good list hygiene practices in order to help stay off of industry blocklists. Comcast uses Spamhaus Zen and RPBL blocklists.
• Try to avoid large, abnormal spikes in sending volume as it may be perceived as spam.
• Secure your systems to block unauthorized access to hackers. Compromised machines sending spam to Comcast will be blocked.
• Do not send bulk email from a dynamic IP address. Comcast does not accept email from a dynamic IP address. Only send to Comcast using a shared or dedicated IP address.
• Read Comcast’s SMTP error codes for clues on delivery problems. The error codes can provide information to help you troubleshoot and fix the problem.
• Send relevant, engaging email to Comcast subscribers. Engagement factors into your sending reputation.
• Warm up new IP addresses slowly in order to build a sending reputation. New IP addresses that are not warmed up properly will be throttled and blocked.
• Respect Comcast’s resources to avoid throttling when sending over IPv4 and IPv6 addresses. Comcast’s suggested connection and throughput settings are:
• 25 simultaneous connections per sending IP address.
• 100 recipients per connection.
• Comcast only allows 1000 messages an hour per netblock for email sent from an IPv6 address.
• Comcast throttling is based on sender reputation and authentication for email sent from an IPv4 address (see below).

• Your Sender Score can influence Comcast’s throttle rate. Do not exceed the maximum recipients per hour for your IP with the corresponding Sender Score below.

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