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.
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.
The 10 most common errors returned by Comcast / Xfinity mail servers. Critical (Tier 1) codes appear first.
If your IP or domain is blocked by Comcast (SC-001 or SC-002), you can request a delist review:
Common questions about Comcast email SMTP errors.
Comcast leans heavily on IP reputation as its primary filter, and it is known for being aggressive about it — even blocking 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.
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.
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.
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.
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