When sending an email, you will sometimes receive an error message with your own code. Here is an explanation of the codes, as well as recommendations for correcting each error.
Note. If you need to contact the email administrator, be sure to include an error message containing important information.
Error codes and messages
ES000001
You have been blocked from sending to the Comcast network because we have determined that you are sending email from a dynamic/residential IP address in the Comcast domain. Comcast does not allow subscribers to send email from mail servers other than smtp.comcast.net.
I don’t think my email is coming from a dynamic/residential source.
Your IP space is for Comcast home services. You can send or receive email through Comcast mail servers using our webmail application or by setting up your email client to connect properly. If you need to run your own mail server, please contact our business services organization at [email protected].
How to remove your IP from the black list?
Since you are connecting through Comcast’s residential IP space, this restriction cannot be lifted at this time. See the instructions above for how to send emails without removing this block.
ES000010
You have been blocked from sending to the Comcast network because we have determined that you are sending email from a dynamic/residential IP address. Comcast does not allow residential/dynamic IP addresses to connect directly to our mail servers. In most cases, Comcast will classify your mail server’s IP address as dynamic if the reverse DNS doesn’t follow the standard static naming conventions.
How to remove your IP from the black list?
It’s important to ask your email provider to check your email server settings to make sure it’s set up correctly so Comcast doesn’t block you in the future. When you contact your email administrator, you should include the error message contained in the email that alerted you to this problem. This error message contains important information that will help your email administrator resolve this issue.
If you need further assistance, please contact Customer Security at 888-565-4329.
BL000000
Our filters determined that an email from your mail server was sent in a spam pattern. To protect subscribers, your mail server has been blocked from sending email to the Comcast network. Mail servers are often used by many users, so it could be that the other party using your mail server sent spam even if you didn’t.
How to remove your IP from the black list?
It is important that your email administrator is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email administrator, you should include the error message contained in the email that alerted you to this problem. This error message contains important information that will help your email administrator resolve this issue. Remove requests can be sent to postmaster.comcast.net/block-removal-request.html and will require the IP address of the blocked mail server. Requests submitted through this form are monitored 24 hours a day, seven days a week to ensure a timely response.
BL000001
Your email could not be delivered because your sending IP address is listed in Spamhaus Zen. This is a list of IP addresses that have been detected as spam and also contains dynamically assigned IP addresses that should not send emails directly to emails. mail servers. Comcast uses these lists to protect its subscribers from spam.
How to fix the crash?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve this issue. To resolve this issue, you or your email service provider can visit the Spamhaus website and request removal.
BL000010
Your email could not be delivered because your email provider is listed in Cloudmark Sender Intelligence (CSI). CSI is a list of IP addresses that have been detected as spam. Comcast uses this list to protect its subscribers from receiving spam.
How to fix the crash?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve this issue. To resolve this issue, you or your email service provider can visit the CSI website and request deletion.
BL000011
Your email could not be delivered because your sending IP address is listed in both Cloudmark Sender Intelligence (CSI) and Spamhaus Zen. Both lists are lists of IP addresses that have been detected as spam and also contain dynamically assigned IP addresses to which emails should not be sent. directly to mail servers. Comcast uses these lists to protect its subscribers from spam.
How to remove the lock?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve this issue. To resolve this issue, you or your email service provider can visit the CSI website and the Spamhaus website to request deletion.
BL000100
Your email could not be delivered because your email provider is blacklisted by the Return Path reputation network. This is a list of IP addresses that have been detected as spam. Comcast uses this list to protect its subscribers from receiving spam.
How to remove the lock?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve this issue. You or your email service provider can visit the Return Path website to request deletion.
BL000101
Your email could not be delivered because your email provider is blacklisted by the Reverse Path Reputation Network and also by Spamhaus Zen. These are lists of IP addresses that have been detected as spam and also contain addresses of Dynamically Assigned IP Addresses that should not send emails directly to mail servers. Comcast uses these lists to protect its subscribers from spam.
How to remove the lock?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve this issue. To resolve this issue, you or your email provider can visit the Return Path website and the Spamhaus website to request deletion.
BL000110
Your email could not be delivered because your email provider is blacklisted in both the return path reputation network and Cloudmark Sender Intelligence (CSI). Both are lists of IP addresses that have been detected as spam and also contain dynamically assigned IP addresses that should not send emails directly to mail servers. Comcast uses these lists to protect its subscribers from spam.
How to remove the lock?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve the issue. To resolve this issue, you or your email service provider can visit the Return Path website and the CSI website to request deletion.
BL000111
Your email could not be delivered because your email provider is listed in the Return Path Reputation Network, Cloudmark Sender Intelligence (CSI) and Spamhaus Zen blacklists. These are lists of IP addresses that have been detected as spam and also contain a dynamically assigned IP address. addresses that should not send emails directly to mail servers. Comcast uses these lists to protect its subscribers from spam.
How to remove the lock?
It is important that your email provider is aware of your outgoing spam issue so that Comcast does not block you in the future. When you contact your email provider, you should include the error message contained in the email that alerted you to this issue. This error message contains important information that will help your email service provider resolve this issue. To resolve this issue, you or your email service provider can visit the Return Path website, the CSI website, and the Spamhaus website to request deletion.
DM000001
This message was rejected by the Comcast mail servers due to the sending domain’s published DMARC policy. The sending domain has published a DMARC reject policy, and the message failed DKIM or SPF authentication for the domain. For more information about DMARC visit http://dmarc.org.
If you need further assistance, please contact Customer Security at 888-565-4329.
RL000001
Comcast has set a rate limit for all incoming email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any system affected by this rate limit will receive a 4xx (temp-fail) message during the SMTP transaction. This message is intended to tell the sending server to try again later to deliver your email.
Our fee cap policy is based on the sender reputation provided by SenderScore as well as the authentication of the sending server, regardless of the number of domains on that server.
| Recipients per Hour | SenderScore* |
| 120 | N/A |
| 1,200 | 0 – 15 |
| 3,600 | 16 – 25 |
| 6,000 | 26 – 30 |
| 12,000 | 31 – 50 |
| 24,000 | 51 – 70 |
| 42,000 | 71 – 85 |
| 60,000 | 86 – 100 |
| *Subject to successful authentication |
If rate limiting is affecting your ability to deliver email, reducing your server’s retry interval should resolve the issue.
If you need further assistance, please contact Customer Security at 888-565-4329.
RL000002
Comcast has set a rate limit for all incoming email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any system affected by this rate limit will receive a 4xx (temp-fail) message during the SMTP transaction. This message is intended to tell the sending server to try again later to deliver your email.
Our fee cap policy is based on sender reputation and volume metrics provided by Cloudmark, regardless of the number of domains on that server.
New IP addresses that are expected to send a significant amount of email to Comcast subscribers should start sending small volumes of email, increasing over time. As your IP sends more emails, your limit will increase daily.
If rate limiting is affecting your ability to deliver email, reducing your server’s retry interval should resolve the issue.
If you need further assistance, please contact Customer Security at 888-565-4329.
RL000003
Comcast has set a rate limit for all incoming email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any system affected by this rate limit will receive a 4xx (temp-fail) message during the SMTP transaction. This message is intended to tell the sending server to try again later to deliver your email.
This fee cap policy is based on historical volumes and the quality of those volumes. The limits applied are independent of the number of domains originating from a given IP address. This policy must apply to both IPv4 and IPv6.
New IP addresses that are expected to send a significant amount of email to Comcast subscribers should start sending small volumes of email, increasing over time. As your IP sends more emails, your limit will increase daily.
If rate limiting is affecting your ability to deliver email, reducing your server’s retry interval should resolve the issue.
If you need further assistance, please contact Customer Security at 888-565-4329.
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. |
Mail servers are often used by many users, so it could be that the other party using your mail server sent spam even if you didn’t. Let your email administrator know about your outgoing spam issue so we don’t block you in the future. Remove requests can be sent to http://postmaster.comcast.net/smtp-error-codes.php and will require the IP address of the blocked mail server (you can get this from your email administrator).
Below is an example error message and where you can find the IP address required for your takedown request.
