Industry's Deepest Blocklist Detection
A single blocklisted URL in your email can send your entire campaign to spam. Most blocklist checkers do a surface-level lookup. Postmaster+ goes deeper — detonating links, following every redirect, and scanning each hop to find blocklists that others miss entirely.
Why Surface-Level Checks Aren't Enough
Most blocklist tools check a domain against a list and return a yes/no result. But modern email links are complex — tracking redirects, URL shorteners, and third-party analytics create multi-hop redirect chains. A blocklist hit on any hop in the chain can trigger spam filtering, and surface-level tools never see those intermediate hops.
A competitor's tool would only check the original URL. Our API follows the full redirect chain and scans every domain and IP at each hop.
Core Capabilities
Link Detonation
Every submitted URL is "detonated" — actually opened and executed in a controlled environment. This catches JavaScript redirects, meta refreshes, and server-side redirects that static URL parsing misses. If a link ultimately leads to a blocklisted domain, we find it.
Full Redirect Chain Analysis
We follow every hop in a redirect chain and check each intermediate domain and IP against blocklists. Tracking pixels, URL shorteners (bit.ly, t.co), and ESP click-tracking links all create redirect chains — any hop can be the one that triggers spam filtering.
Impactful Blocklists Only
We don't check hundreds of obscure, unmaintained blocklists that no mailbox provider uses. Postmaster+ scans only the blocklists known to affect email delivery — Spamhaus (SBL, DBL), SURBL, URIBL, Barracuda, and others that Gmail, Outlook, and Yahoo actively query. No false alarms from irrelevant lists.
API Quick Start
The Blocklist API is asynchronous — submit URLs, get a scan ID, then poll for results. This allows deep analysis without timeout constraints.
1. Start a scan
curl -X POST "https://postmasterplus.app/api/v1/blocklist/scan/start" \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"urls": [
"https://example.com/track?id=123",
"https://bit.ly/abc123"
]
}'2. Check scan status
curl -X GET "https://postmasterplus.app/api/v1/blocklist/scan/status/{scan_id}" \
-H "Authorization: Bearer YOUR_API_KEY"When to Use the Blocklist API
Pre-send email content scanning
Scan every link in your email template before sending. A single blocklisted URL — even in a footer or tracking pixel — can cause the entire email to be filtered to spam.
Automated CI/CD pipeline checks
Integrate blocklist scanning into your deployment pipeline. Automatically check all URLs in email templates before they go to production.
Third-party link validation
When your emails include links to partner sites, affiliate URLs, or user-generated content, scan them to ensure they haven't been blocklisted since you last checked.
Incident investigation
When deliverability suddenly drops, scan all links in your recent campaigns to identify if a newly blocklisted URL is the cause.
ESP platform safety
Email service providers can scan customer content before sending to protect shared IP reputation from blocklisted links.