When sending email, you'll occasionally receive an error message with its own code. Here's an explanation of the codes — along with a recommendation on how to fix each error.
Note: In the event you need to contact your email administrator, be sure to include your error message, which contains 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 within the Comcast domain. Comcast does not allow subscribers to send email from email servers other than smtp.comcast.net.
I don't believe my email is originating from a dynamic/residential source.
Your IP space is designated for residential Comcast service. You can send or receive email through the Comcast email servers by using our webmail application or by configuring your email client to connect properly. If you need to run your own email server, please contact our Commercial Services organization at support_biz@cable.comcast.com.
How do I get my IP removed from the blocklist?
Since you are connecting through Comcast residential IP space, this restriction cannot be lifted at this time. Please see the above instructions on "how to" options to send email without lifting this block.
ES000010
You have been blocked from sending to the Comcast network because we have determined that you are sending email from dynamic/residential IP space. Comcast does not allow residential/dynamic IP addresses to directly connect to our email servers. In most cases, Comcast will classify your email server's IP address as dynamic if the reverse DNS does not comply with standard static naming conventions.
How do I get my IP removed from the blocklist?
It is important that you ask your email provider to review the settings of your email server to ensure it is set up correctly so that it is not blocked by Comcast in the future. When contacting your email administrator, you should include the error message contained in the email which alerted you to this problem. This error message contains important information to help your email administrator resolve this issue.
If you require additional assistance, please contact Customer Security Assurance at 1-888-565-4329.
BL000000
Our filters have determined that email from your email server has been sent in patterns which are characteristic of spam. In an effort to protect subscribers, your email server has been blocked from sending email to the Comcast network. Mail servers are typically shared by many users so it may be the case that another party using your email server has sent spam, even if you have not.
How do I get my IP removed from the blocklist?
It is important that your email administrator is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email administrator, you should include the error message contained in the email which alerted you to this problem. This error message contains important information to help your email administrator resolve this issue. Removal requests can be submitted at postmaster.comcast.net/block-removal-request.html and will require the IP address of the blocked email 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 on the Spamhaus Zen. This is a list of IP addresses which have been detected as sending spam and also contains dynamically assigned IP addresses which are not supposed to send email directly to email servers. Comcast uses these lists to protect its subscribers from receiving spam.
How do I remediate the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve this issue. To address this issue, either you or your email provider can visit the Spamhaus website in order to request removal.
BL000010
Your email could not be delivered because your email provider is listed on the Cloudmark Sender Intelligence (CSI). CSI is a list of IP addresses which have been detected as sending spam. Comcast uses this list to protect its subscribers from receiving spam.
How do I remediate the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve this issue. To address this issue, either you or your email provider can visit the CSI website in order to request removal.
BL000011
Your email could not be delivered because your sending IP address is listed on both Cloudmark Sender Intelligence (CSI) as well as the Spamhaus Zen. Both are lists of IP addresses which have been detected as sending spam and also contain dynamically assigned IP addresses which are not supposed to send email directly to email servers. Comcast uses these lists to protect its subscribers from receiving spam.
How do I remove the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve this issue. To address this issue, either you or your email provider can visit the CSI website and the Spamhaus website to request removal.
BL000100
Your email could not be delivered because your email provider is listed on the Return Path Repuation Network Blocklist. This is a list of IP addresses that have been detected as sending spam. Comcast uses this list to protect its subscribers from receiving spam.
How do I remove the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve this issue. Either you or your email provider can visit the Return Path website to request removal.
BL000101
Your email could not be delivered because your email provider is listed on the Return Path Reputation Network Blocklist, as well as the Spamhaus Zen. These are lists of IP addresses which have been detected as sending spam and also contain dynamically assigned IP addresses which are not supposed to send email directly to email servers. Comcast uses these lists to protect its subscribers from receiving spam.
How do I remove the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve this issue. To address this issue, either you or your email provider can visit the Return Path website and the Spamhaus website to request removal.
BL000110
Your email could not be delivered because your email provider is listed on both the Return Path Reputation Network Blocklist, as well as the Cloudmark Sender Intelligence (CSI). Both are lists of IP addresses which have been detected as sending spam and also contain dynamically assigned IP addresses which are not supposed to send email directly to email servers. Comcast uses these lists to protect its subscribers from receiving spam.
How do I remove the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve the issue. To address this issue, either you or your email provider can visit the Return Path website and the CSI website in order to request removal.
BL000111
Your email could not be delivered because your email provider is listed on the Return Path Reputation Network Blocklist, the Cloudmark Sender Intelligence (CSI), as well as the Spamhaus Zen. These are lists of IP addresses which have been detected as sending spam and also contain dynamically assigned IP addresses which are not supposed to send email directly to email servers. Comcast uses these lists to protect its subscribers from receiving spam.
How do I remove the block?
It is important that your email provider is aware of their outbound spam problem to avoid being blocked by Comcast in the future. When contacting your email provider, you should include the error message contained in the email that alerted you to this problem. This error message contains important information to help your email provider resolve this issue. To address this issue, either you or your email provider can visit the Return Path website, the CSI website, and the Spamhaus website in order to request removal.
DM000001
This message has been 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 this message has not passed either DKIM or SPF authentication for the domain. For more information on DMARC, visit http://dmarc.org.
If you require additional assistance, please contact Customer Security Assurance at 1-888-565-4329.
RL000001
Comcast has instituted rate limiting on all inbound email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any systems affected by this rate limit will receive a 4xx message (temp-fail) during the SMTP transaction. This message is designed to instruct the sending server to try again at a later time to deliver its email.
Our rate limiting policy is based on sender reputation, provided by SenderScore, as well as authentication of the sending server, regardless of the number of domains from 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 the rate-limit is impacting your ability to deliver email, shortening your server's delivery retry interval should resolve the issue.
If you require additional assistance, please contact Customer Security Assurance at 1-888-565-4329.
RL000002
Comcast has instituted rate limiting on all inbound email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any systems affected by this rate limit will receive a 4xx message (temp-fail) during the SMTP transaction. This message is designed to instruct the sending server to try again at a later time to deliver its email.
Our rate limiting policy is based on sender reputation and volume metrics, provided by Cloudmark, regardless of the number of domains from that server.
New IP addresses that are expected to send significant amounts of email to Comcast subscribers should start by sending low volumes of mail, increasing as time goes on. As your IP sends more email, its limit will increase daily.
If the rate-limit is impacting your ability to deliver email, shortening your server's delivery retry interval should resolve the issue.
If you require additional assistance, please contact Customer Security Assurance at 1-888-565-4329.
RL000003
Comcast has instituted rate limiting on all inbound email traffic. This step was taken to protect our customers and our email system from unwanted spam and malicious attacks. Any systems affected by this rate limit will receive a 4xx message (temp-fail) during the SMTP transaction. This message is designed to instruct the sending server to try again at a later time to deliver its email.
This rate limiting policy is based on historical volumes and quality of that volume. The limits applied are independent of the number of domains originating from a given IP. This policy should apply to both IPv4 and IPv6.
New IP addresses that are expected to send significant amounts of email to Comcast subscribers should start by sending low volumes of mail, increasing as time goes on. As your IP sends more email, its limit will increase daily.
If the rate-limit is impacting your ability to deliver email, shortening your server's delivery retry interval should resolve the issue.
If you require additional assistance, please contact Customer Security Assurance at 1-888-565-4329.
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 15 MB 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 typically shared by many users so it may be the case that another party using your mail server has sent spam, even if you haven’t. Make your email administrator aware of its outbound spam problem to avoid being blocked by us in the future. Removal 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 of an error message and where you can find the IP address needed for your removal request.
Additional information
Visit our Online Support Center to find common solutions.
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