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Provider deliverability · Comcast / Xfinity (comcast.net)

Why is Comcast blocking my emails?

Samuel Chenard

By Samuel Chenard · CEO & Co-Founder, Palisade · Reviewed September 19, 2026

Comcast rejects mail during the SMTP transaction and returns a coded error such as BL000000 or ES000010. The usual causes are a listing on a blocklist Comcast consults, a missing or failing PTR record, mail sent from dynamic or residential IP space, or rate limiting that Comcast applies harder to unauthenticated senders.

The 30-second check

Comcast blocks by sending IP far more often than by content, and its BL-series codes name the exact list that has you: Spamhaus Zen, Cloudmark Sender Intelligence, the Return Path Reputation Network Blocklist, or the Vade Threat List. Start with the domain your mail leaves from; the blocklist check below resolves it to the sending IPs and reports what each list says about them.

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Why Comcast is blocking your email

Likely causeWhat's happening
Your sending IP is on a blocklist Comcast consultsComcast documents a family of BL-series codes and each one names the list responsible: BL000001 is Spamhaus Zen, BL000010 is Cloudmark Sender Intelligence, BL000100 is the Return Path Reputation Network Blocklist, and BL001000 is the Vade Threat List. Combined codes mean more than one list has you. Comcast's guidance for all of them is the same: get delisted at the source, because Comcast does not lift another operator's listing.
Comcast's own filters saw spam-shaped traffic from your serverBL000000 is Comcast's own block, not a third party's. Comcast's wording is that its filters "have determined that email from your email server has been sent in patterns which are characteristic of spam". It is also the only code Comcast's online removal form accepts, and Comcast notes a shared mail server can be blocked because of a different user on it.
Reverse DNS is missing, generic, or failingComcast performs an rDNS check on the sending IP and says that if the server has no PTR record and a properly configured MX or A record, the connection will not be accepted. A failed lookup returns 421 - [Reverse DNS failure : Try again later]; no PTR record at all returns 554 - [PTR lookup failure]. Comcast also classifies an IP as dynamic when the reverse DNS does not follow standard static naming conventions.
You are sending from dynamic or residential IP spaceComcast does not accept mail from dynamic IP space and publishes the CIDR ranges it treats that way. ES000010 is the general dynamic-space block. ES000001 is narrower: it means you are sending from residential Comcast IP space, and Comcast states that restriction cannot be lifted, so the fix is to relay through smtp.comcast.net or move the mail to a commercial connection.
You are being rate limited, not blockedThe RL-series codes are 4xx temporary failures, so the mail is deferred rather than lost. Comcast publishes a table of recipients per hour tied to SenderScore, from 120 per hour with no score up to 60,000 at a score of 86 to 100, and marks the whole table "subject to successful authentication". RL000020 is the bucket Comcast applies to unauthenticated email specifically, and IPv6 senders are capped at 1,000 messages an hour per netblock.
Your own DMARC policy is rejecting the messageDM000001 is Comcast enforcing your published policy, not its own. Comcast returns it when the sending domain publishes a DMARC reject policy and the message passed neither DKIM nor SPF for that domain. The block is a symptom of a sender that was never authorised in your records, not a reason to weaken the policy.
You crossed a connection, session, or recipient limitComcast publishes hard numbers: 25 simultaneous connections per sending IP, 1,000 messages per session, 100 recipients per message, and a 15 MB message size cap. It also treats "Not our customer" non-delivery notices as unsubscribe requests, and blocks IPs that keep mailing invalid recipients, which it describes as directory harvest behaviour.

Check the public signals before changing settings

The free check above gives you a fast public baseline for how Comcast sees your mail. It does not replace the provider's private reputation or placement data, but it tells you whether the problem is visible from outside before you edit a sending platform.

Palisade Email Security Score result showing public DMARC, SPF, DKIM, MX, and reputation checks for a non-sensitive test domain.
Source: Palisade, “Email Security Score”, checked 2026-07-29. First-party public tool result for a non-sensitive test domain; it checks public DNS and reputation signals, not private mailbox placement.

How to fix it, step by step

  1. Check the sending IP against the lists Comcast uses

    Run the free check above, or open the blocklist checker page directly against the IP your mail actually leaves from, which is usually your provider's outbound IP rather than your website's. Spamhaus Zen, Cloudmark CSI, Return Path, and Vade are the four Comcast names in its BL codes, so a hit on any of them explains the rejection on its own.

  2. Read the code in the bounce before changing anything

    Comcast puts a specific code in the rejection and publishes what each one means. A BL code is a listing, an ES code is IP space, an RL code is a deferral you should retry rather than escalate, and DM000001 is your own DMARC policy. Matching the code first stops you rewriting DNS for a problem that lives on a blocklist.

  3. Fix reverse DNS on every sending IP

    Set a PTR record that resolves back to a hostname with a matching A record, and use static naming rather than a generic ISP-assigned pattern. Comcast checks this on connection and returns 421 or 554 when it fails, and it uses the same signal to decide whether your IP looks dynamic.

  4. Authenticate the mail with SPF and DKIM

    Comcast's published sender requirements urge every sender to use TLS 1.2 or 1.3 and some form of message authentication, and say failure to do so will affect throughput, reputation, and placement. Its rate-limit table is explicitly conditional on successful authentication. Verify both records with the free SPF and DKIM checkers.

  5. Submit Comcast's block removal request, but only for BL000000

    Comcast's form handles that one code and points every other block back at the list that issued it. Comcast says a request typically takes under 30 minutes to lift, and limits submissions to five per IP a day and 20 IPs a day per requester. Fix the outbound spam source first or the block returns.

  6. Enrol in Comcast's feedback loop

    Sign up at feedback.comcast.net as the party responsible for the sending server. Comcast sends an ARF report, with its customers' addresses scrubbed, every time a user hits "This is Spam", and supports both IP-based and DKIM-based registration so you can suppress the recipient immediately.

  7. Re-test, then retry rather than escalate

    Send to a comcast.net mailbox and read the result. RL and 421 codes are temporary and clear on retry once reputation improves, so shortening your retry interval is Comcast's own suggested response. Escalate only when a 5xx block persists after the listing is cleared and the records are correct.

Related free tools: IP reputation · DMARC checker · SPF checker · DKIM checker

If you send in volume: Comcast's published rules

Comcast publishes a short "Sender Requirements" statement rather than a Gmail-style or Yahoo-style bulk sender programme, and sets no complaint-rate ceiling. Comcast "strongly urges all senders to utilize TLS (v1.2 or v1.3) and some form of message authentication (DKIM or SPF) when sending messages to comcast.net recipients", and says failure to do so will impact throughput, reputation, and placement. That is a floor of SPF or DKIM, not both, and no DMARC requirement. The nearest thing to a volume rule is the rate-limit table, where recipients per hour scale with SenderScore and the whole table is marked subject to successful authentication. Per Comcast's postmaster documentation, checked 2026-09-02.

Check your standing with Comcast

Bounce codes you may be seeing

A block from Comcast surfaces as a specific SMTP code. Some lists below also carry codes returned by the systems receiving your mail; each line names its issuer. Match yours below; where a code has a full guide, it is linked.

The real root cause: unenforced authentication

Comcast's codes look like an infrastructure problem, and the first fixes usually are: a PTR record, a delisting, a move off residential IP space. But authentication decides how much of Comcast's leniency you get afterwards. The published rate-limit table is marked subject to successful authentication, RL000020 exists purely for unauthenticated mail, and Comcast's own sender requirements say that skipping DKIM or SPF will cost you throughput, reputation, and placement. DM000001 closes the loop: Comcast honours DMARC policies, so an unauthorised sender using your domain gets rejected by name. The durable position is a domain where every legitimate service is covered by SPF and signs with DKIM, where DMARC aligns with the From address, and where the policy has been walked from p=none up to p=reject so that impostor traffic never reaches a Comcast filter with your domain on it. Monitoring tells you which senders are failing. Enforcement is what stops the reputation you rebuilt from being spent by someone else.

DMARC software that does the work

Palisade's AI agent takes domains all the way to enforcement: hosted SPF, DKIM, DMARC, and MTA-STS records on paid plans, DMARC reports monitored continuously, and every policy step drafted for your approval on the way to p=reject. The full product is open for a 15-day trial, and nothing is charged until it ends.

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15-day full-product trial. Nothing is charged until it ends.

Fixing this across every client domain

Comcast grades each sending IP and each domain separately, and its block removal form caps you at five requests per IP a day and 20 IPs a day, which turns a fleet-wide listing into a queue rather than a fix. If your clients share an outbound relay, one compromised mailbox blocks everyone on that IP at once. Palisade makes the domain side of that a single workflow: it hosts and manages SPF, DKIM, DMARC, and MTA-STS records for every client domain, reads aggregate reports to name the senders that are failing before a receiver starts deferring them, and proposes each step toward p=reject with your team approving every change. ConnectWise, HaloPSA, and Autotask integrations put it in your PSA, pricing is per client domain with rates that improve as the portfolio grows, and your own MSP domain is a free NFR domain to run the process on first.

Questions readers ask

Frequently asked questions

What does BL000000 mean from Comcast?

BL000000 is Comcast's own block, applied when its filters judge that your server has sent mail in patterns characteristic of spam. It is the only code Comcast's online removal form accepts. Comcast notes that shared mail servers get blocked for another user's behaviour, so check every sender on the IP before requesting removal.

How do I get my IP removed from Comcast's blocklist?

Comcast's removal form handles BL000000 only, and Comcast says a request typically takes under 30 minutes to lift. Submissions are capped at five per IP a day and 20 IPs a day. Every other BL code points at a third-party list, so you request removal from Spamhaus, Cloudmark, Return Path, or Vade directly.

Does Comcast require SPF, DKIM, or DMARC?

Comcast requires none of the three outright. Its published sender requirements urge TLS 1.2 or 1.3 and some form of message authentication, DKIM or SPF, and warn that skipping it affects throughput, reputation, and placement. Comcast does enforce your own DMARC policy, returning DM000001 when a message fails it.

What is Comcast's sending rate limit?

Comcast publishes recipients per hour against SenderScore: 120 with no score, 1,200 at a score of 0 to 15, rising to 60,000 at 86 to 100, and marks the table subject to successful authentication. Separately it allows 25 simultaneous connections per IP, 1,000 messages per session, and 100 recipients per message.

Why is Xfinity blocking my emails but Gmail is not?

Xfinity and Comcast are the same mail system, and it blocks on signals Gmail weighs differently. Comcast rejects dynamic IP space outright, checks reverse DNS on connection, and consults Cloudmark, Return Path, and Vade alongside Spamhaus. A clean Gmail result therefore proves nothing about a Comcast block.

What does an RL code from Comcast mean?

An RL code is rate limiting, returned as a 4xx temporary failure rather than a rejection, so the message stays queued and retries normally deliver. RL000020 is the bucket Comcast applies to unauthenticated mail. Comcast suggests shortening your retry interval, and warns that sending faster on a poor reputation makes things worse.

Does Comcast publish a spam complaint rate limit?

Comcast publishes no complaint-rate ceiling, unlike Yahoo's 0.3% figure. It does run a feedback loop at feedback.comcast.net that sends an ARF report for every message a Comcast user marks as spam, with customer addresses removed, and supports both IP-based and DKIM-based registration.

Why is Comcast rejecting my mail with DM000001?

DM000001 means Comcast is applying your own domain's DMARC policy. The From domain publishes a reject policy and the message passed neither SPF nor DKIM for it. Find the service that sent it, authorise it in SPF or have it sign with DKIM, and keep the policy rather than relaxing it.

Sources and last verified

Every claim about Comcast on this page links to the source it came from, each with the date it was last checked. Provider policies change; if a detail looks off, the linked source is authoritative.

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