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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. |
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.
