Published May 1, 2026 · Last verified June 1, 2026 · Maintained by TamingDNS

↩️ Comcast / Xfinity Email SMTP Error Code Directory

Comcast (the company behind Xfinity consumer email) has a reputation for being one of the stricter inbound filters in the industry, particularly around IP reputation. Their bounce messages mix standard SMTP enhanced codes with Comcast-specific policy codes like SC-001 and DY-001. This directory explains what each one means and what you need to do to fix it.

Provider

Comcast / Xfinity / @comcast.net

Key Identifiers

SC-001 · SC-002 · DY-001 · RL-001

MX Hostname Pattern

*.comcast.net

How to Identify a Comcast Bounce

Comcast bounces arrive from [email protected]. The Remote-MTA contains a comcast.net hostname. The diagnostic text often includes a Comcast-specific policy code (like SC-001 or DY-001) and a server identifier in parentheses.

550 SC-001 (COL004-MC3F33) Unfortunately, messages from [198.51.100.1] weren't
sent. Please contact your Internet service provider since part of their network
is on our block list. IB421

SC-001 = IP block. SC-002 = domain block. DY-001 = content policy. RL-001/RL-002 = rate limit. The alphanumeric server identifier (e.g. COL004-MC3F33) is Comcast's internal reference and can be quoted when contacting Comcast Postmaster.

Comcast Error Codes

The 10 most common errors returned by Comcast / Xfinity mail servers. Critical (Tier 1) codes appear first.

🔴 550 5.7.1 CRITICAL

IP Address Blocked by Policy

Your sending IP address is blocked by Comcast's reputation filters. This is the most common hard block for new or low-reputation senders to Comcast / Xfinity mailboxes.

550 SC-001 (COL004-MC3F33) Unfortunately, messages from [198.51.100.1] weren't sent. Please contact your Internet service provider since part of their network is on our block list.

🔴 550 5.7.1 CRITICAL

Domain Blocked by Policy

Your sending domain is blocked at the Comcast gateway. Domain blocks can result from sending to spam traps, high complaint rates, or a poor domain reputation score.

550 SC-002 (COL004-MC3F33) Unfortunately, messages from your domain weren't sent. Please contact your Internet service provider since part of their network is on our block list.

🔴 550 5.7.1 CRITICAL

Content Policy Violation

The message content triggered Comcast's spam or content filters. Review your email for spam-like keywords, excessive links, or suspicious attachments.

550 DY-001 (COL004-MC2F22) The message was rejected because it appears to contain content that Comcast does not permit in email.

🟡 421 4.7.0 CRITICAL

Rate Limited: Too Many Messages

You are sending too many messages per hour to Comcast. Throttle your sending rate and space messages out. Your mail server will retry automatically.

421 RL-001 (COL004-MC1F11) Too many messages sent from your IP. Please slow down your sending rate.

🟡 421 4.7.0 CRITICAL

Rate Limited: Too Many Connections

Your sending IP is opening too many concurrent SMTP connections to Comcast mail servers. Reduce to 1-2 concurrent connections per IP.

421 RL-002 (COL004-MC1F11) Too many concurrent connections from your IP.

🔴 550 5.7.23 CRITICAL

SPF Validation Failed

Your sending IP is not authorised by your domain's SPF record. Add the sending IP or use an include mechanism for your ESP to your SPF record.

550 SPF-001 Message rejected because SPF check failed for domain example.com.

🔴 550 5.1.1 CRITICAL

Mailbox Does Not Exist

The recipient's Comcast / Xfinity mailbox does not exist. The address contains a typo or the account has been closed. Remove from your list immediately.

550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in virtual mailbox table.

🔴 550 5.2.2

Mailbox Over Quota

The recipient's Comcast inbox is full. This is often a sign of an abandoned account. Retry in a few days; if it continues bouncing, treat the address as inactive.

550 5.2.2 <[email protected]>: Recipient address rejected: Mailbox over quota.

🔴 550 5.7.26 CRITICAL

DMARC Policy Violation

Your message failed DMARC alignment checks. Ensure DKIM signing aligns with your From domain, and that your SPF record covers your sending IP.

550 5.7.26 This message does not pass authentication checks (SPF and DKIM both do not pass).

🟡 421

Greylisting / Temporary Deferral

Comcast is temporarily deferring your message, often due to greylisting. Your mail server will retry automatically. Most legitimate mail is accepted on the second attempt.

421 Service temporarily unavailable. Please try again later.

Getting Delisted from Comcast

If your IP or domain is blocked by Comcast (SC-001 or SC-002), you can request a delist review:

Ensure your SPF and DKIM are correctly configured before requesting review
Keep your spam complaint rate below 0.1%
Remove invalid addresses and spam trap hits from your list
Contact Comcast Postmaster with your sending IP, domain, and volume information

📖 Comcast Postmaster →

Frequently Asked Questions

Common questions about Comcast email SMTP errors.

Why does Comcast block legitimate senders with SC-001?

Comcast leans heavily on IP reputation as its primary filter, and it is known for being aggressive about it. It will block entire IP ranges that belong to major cloud providers and email service platforms. An SC-001 usually means your sending IP is new, was previously associated with spam, or has received a notable complaint rate from Comcast users. The fix requires a dedicated sending IP with a clean history, correct authentication records (SPF, DKIM, DMARC), and a delist request submitted through the Comcast Postmaster portal.

What is the DY-001 content policy error?

DY-001 means something in the message body set off Comcast's content filters. The most common triggers are: too many links, subject lines full of capital letters or phrases like "free" and "click here", HTML that is mostly images with very little text, or hidden or malformed elements in the code. The quickest way to diagnose this is to run your email through a spam score checker before sending. Simplifying your content almost always helps.

How long does a Comcast IP block last?

Unlike some providers where blocks expire after a few days, Comcast IP blocks are not automatic. They stick around until someone at Comcast manually reviews your delist request. You need to submit it through the Comcast Postmaster portal and then wait. Response times vary quite a bit: sometimes 24 hours, sometimes several business days. Getting your authentication and sending practices clean before you submit will speed up the review.

Does Comcast enforce DMARC?

Yes. Comcast enforces DMARC for inbound mail to Comcast and Xfinity mailboxes. If your sending domain has a p=reject or p=quarantine policy and your messages fail DMARC alignment, Comcast will act on the policy. A valid SPF record and DKIM signature aligned to your From domain are both essential for consistent delivery to Comcast.