Operational principle: Preserve the exact Comcast SMTP response, validate reverse DNS and authentication, and avoid repeated submissions before traffic is corrected.
1. Capture evidence before changing anything
- Exact SMTP response, enhanced status code and timestamp in UTC.
- Outbound IP, PTR hostname, EHLO name, envelope sender and visible From domain.
- DKIM selector and signing domain; SPF and DMARC alignment result.
- Queue age, retry count, accepted/temporary/permanent failure rates and recent volume change.
- Whether the problem affects one campaign, one domain, one IP or the complete sending platform.
2. Stabilize the sender
- Pause suspicious traffic and secure compromised SMTP/API credentials.
- Remove invalid, inactive and complaint-prone recipients.
- Reduce concurrency and volume only when the evidence indicates reputation or rate pressure.
- Keep transactional and promotional streams separated.
- Do not rotate to new IPs to evade a block; that usually spreads the problem.
3. Verify identity and DNS
PTR(outbound_ip) → mail.example.com
EHLO → mail.example.com
SPF → pass for the envelope domain
DKIM → pass with aligned d= domain
DMARC → pass through SPF or DKIM alignment4. Official resources
Monitoring or sender portal
Use this to review provider-specific reputation or delivery information.
Open official resource ↗Requirements and troubleshooting
Compare the exact failure with the provider’s published requirements.
Read official guidance ↗Support or escalation
Escalate only after the sender is stable and the problem remains reproducible.
Open support channel ↗5. Evidence package for escalation
Subject: Delivery issue from [IP] to [provider]
Sending IP and PTR:
Envelope-from domain:
DKIM signing domain and selector:
Visible From domain:
First observed (UTC):
Latest example (UTC):
Exact SMTP response:
Approximate affected volume:
Corrective actions completed:
Requested review:6. Recovery controls
After acceptance returns, increase volume gradually. Track temporary failures, complaint signals, queue age and authentication results. Stop expansion if negative signals return.