Comcast and Xfinity Bounce Codes: BL000000, RL000001 and 550 Not Our Customer

Comcast bounce codes tell you exactly what went wrong: BL codes are blocks, RL codes are rate limits, and 550 "Not our customer" means the comcast.net address doesn't exist. Comcast publishes these on its email errors support page, written for senders. Blocks and rate limits are fixed on your sending side. Dead addresses are fixed on your list, and that part you can prevent by verifying comcast.net addresses before you send.

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The codes at a glance

From Comcast's support page, read on 9 November 2026, in our words:

Code	Type	What it means
BL000000	Block	Your server sent in patterns that look like spam, so it was blocked
BL000001 to BL001111	Block	Your IP is on one or more outside blocklists
RL000001	Rate limit, 4xx	Limit based on SenderScore reputation and authentication
RL000002	Rate limit, 4xx	Limit based on Cloudmark reputation and volume
RL000003	Rate limit, 4xx	Limit based on your historical volume and quality
DM000001	Reject	Your domain's DMARC policy failed both DKIM and SPF
ES000001, ES000010	Block	Sending from dynamic or residential IP space
550 Not our customer	Bad address	The recipient doesn't exist
550 Too many invalid recipients	Bad list	Too many bad addresses; nobody got the message

The rest of this post goes through each one with a fix.

BL000000: spam-pattern block

Comcast's wording: "Filters found that email from your server was sent in patterns that seem like spam." The server is blocked from sending to the Xfinity network.

It also notes that "Mail servers are often shared by many users, so it is possible someone else using your server sent spam." On a shared sending IP, the block may not be your fault, but it still stops your mail.

What to do:

1. If you use an email provider, send them the full bounce. Comcast asks you to "include all error details."
2. If you run your own server, submit a removal request on Comcast's postmaster site. "The request needs the IP address of the blocked email server."
3. Fix the cause before you resume. Spam-like patterns usually mean a burst of mail to a list with bad addresses, or a sudden jump in volume. Sending the same campaign again after removal tends to get the same block.

BL000001 to BL001111: blocklist codes

These mean your IP is listed on one or more outside blocklists. The digits work like switches, read right to left:

Digit position (from right)	Blocklist
1st	Spamhaus Zen
2nd	CSI
3rd	Return Path
4th	VTL

So BL000001 is Spamhaus Zen only, BL000011 is CSI and Spamhaus Zen, and BL001111 is all four.

What to do: check the IP on those lists with the IP blacklist check, then follow each list's delisting process. The blacklist removal helper walks through the common ones. Comcast's block lifts once the listing is gone. Spam traps and bad addresses are a common reason for listings, so clean the list before you send again.

RL000001, RL000002, RL000003: rate limits

Comcast says: "Xfinity has rate limits on all inbound email traffic." Affected servers "get a 4xx message (temp-fail) during the SMTP transaction," which "tells the sending server to try to deliver the email again later."

These are deferrals, not bounces. The mail is not lost if your server retries.

• RL000001 is based on "Sender reputation provided by SenderScore" and "Authentication of the sending server." Comcast's page lists recipients per hour against SenderScore bands, and the limits it lists apply only after successful authentication. Better reputation gets a higher hourly limit.
• RL000002 is based on Cloudmark reputation and volume. Comcast says new IPs should ramp up slowly.
• RL000003 is based on your historical volume and quality. Again, ramp new IPs slowly.

What to do:

1. Make sure SPF, DKIM and DMARC pass. RL000001's higher limits need authentication. Check your domain with the email domain health checker.
2. Follow Comcast's own tip: "shorten your server's delivery retry interval."
3. Spread the send. Lower your hourly volume to comcast.net addresses. The email send rate calculator helps you plan it.
4. Improve reputation at the source. Hard bounces and complaints lower it. A cleaner list is the steadiest way to raise your limits over time.

DM000001: DMARC rejection

Comcast rejects mail when the sending domain's DMARC policy fails both DKIM and SPF. This usually means a tool sends as your domain without being set up in your DNS.

What to do: check alignment with the DMARC alignment checker. Add every sending service to SPF and set up DKIM for each. Microsoft 550 5.7.509 DMARC covers the same fix for Outlook.

ES000001 and ES000010: residential IP space

These block mail from dynamic or residential IP addresses. ES000010 usually means your reverse DNS doesn't follow static naming conventions.

What to do: send through a proper mail server or email provider, not a home connection, and make sure your sending IP has a PTR record that matches. The reverse DNS lookup shows yours, and reverse DNS and PTR records for email explains what a good one looks like. Comcast also lists 421 and 554 errors for missing or invalid reverse DNS.

550 Not our customer: the address doesn't exist

Comcast's wording: "This error shows when a message is sent to an Xfinity customer that doesn't exist." It adds that it "should be treated as an 'unsubscribe' by the sender, if a bulk mailer."

This is a hard bounce. Retrying won't help.

What to do:

1. Remove the address now and add it to your suppression list.
2. Find the others before they bounce. If one comcast.net address on a list is dead, others from the same source probably are too. Old consumer lists collect closed ISP accounts over the years.
3. Verify before the next send. Check a few addresses with the Comcast email checker, or upload the whole list at /verify.

550 5.1.1 user unknown covers the same problem at other providers.

550 Too many invalid recipients

This is the one that hurts most. Comcast's wording: "This error results when a message is sent to too many invalid recipients. As a result none of the recipients received the message." Its advice: "Please check your distribution list and resend."

Good addresses on the same message miss out because of the bad ones. And a high rate of invalid recipients is the kind of pattern that leads to BL000000.

What to do: verify the list, remove the invalid rows, then resend. Don't resend the same list unchanged.

Comcast's other limits worth knowing

From the same page, in our words:

• More than 25 simultaneous connections from one IP gets a 421.
• More than 1,000 emails in one SMTP session gets a 452.
• More than 100 recipients on one message gets a 452.
• Messages larger than 15 MB get a 552.
• A sending domain with no valid A or MX record gets a 550.

Most email platforms handle these for you. If you run your own server, set your connection and batch limits under them.

Prevent Comcast bounces before the send

Most Comcast problems trace back to the list. Bad addresses cause 550s, too many of them refuse the whole message, and the bounces lower the reputation your rate limits depend on.

1. Pull the comcast.net rows from your list and check them first. They're often the oldest addresses on consumer lists.
2. Verify the full file at /verify. Invalid rows go to suppression; risky rows go into a smaller, slower send.
3. Re-check lists that sat unused for a few months. How often to verify an email list gives a schedule.
4. Read every bounce. Paste one into the bounce message analyzer if the code isn't clear.

If you also send to other US ISP addresses, AT&T, Verizon and SBCglobal email addresses covers those, and Yahoo bounce codes and Outlook bounce codes cover the big webmail providers.

Quick answers

• Is RL000001 a bounce? No. It's a temporary failure, and your server should retry.
• Does a removal request fix BL000000 for good? Only if the cause is fixed. Send the same pattern again and you can be blocked again.
• Can I verify comcast.net addresses? Yes. Use the Comcast email checker for single addresses or /verify for a list.

Try it on your own list

Upload a sample at /verify. The first 100 checks are free with a Google sign-in, no card.

Pro, $39 a month: Unlimited email verification.* Never metered, never cut off. See pricing.

*Fair use: the first 100,000 checks a month at full speed, then slower, never cut off.