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What are Comcast's Mail Delivery Error Codes?

Mail to Comcast is rejected and is returned with an error message. What does this mean?

With more than 14 million high-speed Internet customers, Comcast takes its responsibility to combat spam very seriously. Comcast is aware that a careful balance is required to effectively reduce spam while also protecting Internet users' online experiences.

If you've received a 550 non-deliverable notification from Comcast, it will contain a block code and a link to one of the below FAQs which details the reason for the blocked connection and how to remediate the condition.

Specific Error Codes
Error Code Returned More information
ES000001 This error results when mail delivery is attempted from Comcast's dynamic/residential space. (Learn more)
ES000010 This error results when mail delivery is attempted from external dynamic/residential space. (Learn more)
BL000000 This error results when mail delivery is attempted from addresses that sent mail in patterns characteristic of spam. (Learn more)
BL000001 This error results when mail delivery is attempted from addresses that are listed on the Spamhaus Zen Blocklist. (Learn more)
BL000010 This error results when mail delivery is attempted from addresses that are listed on Cloudmark CSI. (Learn more)
BL000011 This error results when mail delivery is attempted from addresses that are listed on the Spamhaus Zen blocklist and Cloudmark CSI. (Learn more)
BL000100 This error results when mail delivery is attempted from addresses that are listed on the SenderScore blocklist. (Learn more)
BL000101 This error results when mail delivery is attempted from addresses that are listed on the Spamhaus Zen blocklist and the SenderScore blocklist. (Learn more)
BL000110 This error results when mail delivery is attempted from addresses that are listed on the SenderScore blocklist and Cloudmark CSI. (Learn more)
BL000111 This error results when mail delivery is attempted from addresses that are listed on the SenderScore blocklist, Cloudmark CSI and the Spamhaus Zen blocklist. (Learn more)
BL001000 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL. (Learn more)
BL001001 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL and Spamhaus Zen blocklist. (Learn more)
BL001010 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL and Cloudmark CSI. (Learn more)
BL001011 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, Cloudmark CSI and Spamhaus Zen blocklist. (Learn more)
BL001100 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL and SenderScore blocklist. (Learn more)
BL001101 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, SenderScore blocklist and Spamhaus Zen blocklist. (Learn more)
BL001110 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, SenderScore blocklist and Cloudmark CSI. (Learn more)
BL001111 This error results when mail delivery is attempted from addresses that are listed on the TrendMicro MAPS RBL, SenderScore blocklist, Cloudmark CSI and Spamhaus Zen. (Learn more)
RL000001 This error results when mail delivery volumes exceed the hourly limits allowed based on the sending IPs reputation. (Learn more)
Non-Specific Error Codes
Error Code Returned For more information
421 - [Too many sessions opened] Comcast allows 25 simultaneous connections per sending IP address. This error results when that limit is exceeded.
421 - [Reverse DNS failure : Try again later] Comcast requires all sending IP addresses have a valid rDNS. This error results when the lookup failed. This error should be treated as a temp-fail and can be retried.

SERVFAIL response. 99.9% means that the DNS somewhere up the auth tree is misconfigured or down or very badly overloaded.
421 - [Try again later] This error results for several reasons. This error should be treated as a temp-fail and can be retried.
452 - [Too many emails sent on this session] Comcast allows 1000 emails per session. This error results when that limit is exceeded.
452 - [Too many recipients for message] Comcast allows 100 recipients per message. This error results when that limit is exceeded.
550 - [Not our customer] This error results when a message is sent to a non-existent Comcast customer. These errors should be treated as an "unsubscribe" by the sender, if a bulk mailer.
550 - [Your message could not be delivered due to too many invalid recipients] This error results when a message is sent to too many invalid recipients. As a result none of the recipients received the message. Please check your distribution list and resend.
550 - [Invalid sender domain] Comcast requires that all sending domains have a valid A or MX record. This error results when neither record can be found.
550 - [Account not available] This error results when a message is sent to a Comcast account that is currently not available.
552 - [Message size exceeded] Comcast allows messages of 15MB or smaller. This error results when that limit is exceeded.
554 - [PTR lookup failure] Comcast requires all sending mail server IP addresses have a valid PTR record set up. This error results when the lookup failed.

NXDOMAIN response. One of the authoritative servers for the relevant section of the in-addr.arpa DNS tree is saying that there is no PTR record for the given IP address.

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.