554 mcc-ibgw-4 cmsmtp Connection RejectedWhat does 554 mcc-ibgw or mdc-ibgw cmsmtp Connection Rejected mean?
mcc-ibgw and mdc-ibgw are Comcast’s Memphis and DC inbound border gateways respectively. They sit at the network edge of the Comcast mail infrastructure and perform connection-level filtering before passing accepted sessions to the downstream impin proxies. A 554 Connection Rejected at this level means cmsmtp refused at the TCP or initial SMTP greeting stage, before EHLO or MAIL FROM. The cause is almost always IP-level: DNSBL match, internal block list entry, or PTR validation failure. The fix is identical to the broader cmsmtp ecosystem: clear public DNSBLs, fix authentication, contact Comcast postmaster.
Is this a soft or hard bounce?
Connection-level rejection means cmsmtp made the decision before even reading your message. This pattern strongly indicates an IP reputation problem rather than a content or authentication issue.
Common causes
Listings on Spamhaus, Abusix, or Barracuda propagate into cmsmtp. Clear those first.
cmsmtp checks PTR at connection time. Missing or generic PTR triggers immediate rejection. See 554 bosimpinc Reverse DNS.
Comcast may have added the IP based on prior complaints or spam trap hits.
How to fix it
Run dig -x <ip> and ensure PTR resolves forward via dig <hostname>. Both must return the same IP.
Address any Spamhaus, Abusix, or Barracuda listings. cmsmtp typically respects upstream delistings within 24 to 48 hours.
Enroll at feedback.comcast.net to track complaints.
Use postmaster.comcast.net with remediation evidence.
Provider-specific notes
| Hostname | Location |
|---|---|
| mcc-ibgw-1 through mcc-ibgw-8 | Memphis inbound border gateway cluster. |
| mdc-ibgw-1 through mdc-ibgw-8 | Washington DC inbound border gateway cluster. |
| Generic cmsmtp Connection Rejected | Connection-layer rejection across any Comcast ingress cluster. Same root cause analysis applies. |
Related error codes
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About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
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