Blog/
SMTP 550 blacklist errors: how to check and recover
What SMTP 550 messages about blocked or listed IPs mean, how to confirm a DNSBL hit, and the order of fixes that restore delivery.
A remote mail server may reject your message with SMTP 550 (or 554) text that mentions blocked, listed, or spam. Often the connecting IP — not your From: domain alone — is on a DNSBL or the provider’s internal reputation list.
Identify the sending IP
Use the IP that appears in the bounce’s remote response or in your outbound gateway logs. Shared hosting may mean a neighbor triggered the listing. Check that IP on major DNSBLs before opening random “delist for pay” sites.
Confirm whether public block lists currently flag the IP.
Recovery order that usually works
- Stop the abuse source (compromised form, open relay, hacked plugin)
- Request delisting using the list operator’s official process
- Confirm PTR / FCrDNS on the sending IP
- Align SPF and DMARC with the same sending hosts
- Warm volume slowly after clearance — do not blast immediately
When lists are clean but mail still fails
Providers also use private reputation, domain age, content filters, and recipient engagement. Clean DNSBLs are necessary, not always sufficient. Authentication and consistent sending identity still matter.