All email must comply with all relevant RFCs.

Comcast performs an rDNS check on the IP of the sending server. If the sending server does not have a PTR record and a MX or an A record set up properly, the connections will not be accepted.

Comcast consults several DNSBLs including Spamhaus Zen, and ReturnPath. Being listed on any reputable, widely used, DNSBL is likely to cause your email to be blocked not only by Comcast, but other ISPs as well.

A large number of undeliverable emails sent to Comcast will result in a sending IP being blocked. All “Not Our Customer” NDNs should be treated as an unsubscribe request.

Enforce AUPs and mitigate compromised accounts and machines. Dictionary or Directory Harvest attacks are treated seriously and will quickly lead to the sending IP being blocked.

Dynamic IP space is a common haven for compromised machines used in bot networks. As a result, in keeping with industry best practices, Comcast does not accept mail from dynamic IP space. If email appears to be coming from dynamic IP space then it will not be accepted.

Overall IP reputation is key to successfully sending to Comcast. If Comcast, or another reputable organization repeatedly receives spam, spyware or other malicious attacks, phishing efforts or other deceptive emails from a certain IP address, Comcast is more likely to block future messages from that IP address. Ensure the overall sending reputation of the sending mail system is as clean as possible.

Most non-delivery notifications Comcast returns contain links to FAQs which explain why the block occurred and instruct in how to remediate the issue. Take the time to review any errors; it is likely they hold the key to a quick remediation.
Visit our error messages page

Comcast allows 25 simultaneous connections per sending IP address.
Comcast allows 100 recipients per message.
Comcast allows a throttle rate based on your sender reputation and authentication.