Lob API
Lob API — programmatic direct mail and address verification — send postcards, letters, and checks via physical mail, and validate US/international addresses from agent workflows.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
API keys are test/live separated — strong pattern for preventing accidental production sends. TLS enforced. SOC2 Type II certified. CCPA compliant.
⚡ Reliability
Best When
Agent workflows need to trigger physical mail delivery programmatically — postcards, letters, or checks — with real-time USPS address validation.
Avoid When
You need digital communication or near-real-time notification — physical mail has inherent delivery lag.
Use Cases
- • Agents sending physical postcards or letters as part of customer outreach campaigns
- • Mailing verification codes or documents that require physical delivery (legal notices, checks)
- • Address standardization and validation before storing or using addresses in agent pipelines
- • Printing and mailing invoices, statements, or reports generated by agents
- • CASS-certified address verification for USPS deliverability checking in real time
Not For
- • Digital messaging — Lob is for physical mail only; use email/SMS for digital
- • International mail at scale — Lob's address verification is strongest for US addresses
- • High-frequency real-time triggering — physical mail has 2-5 day delivery lead times
Interface
Authentication
API Key via HTTP Basic Auth (key as username, empty password). Test and live API keys separate — use test key to preview without incurring costs.
Pricing
Test API key allows full API exploration without sending real mail. Production use requires payment method. Volume discounts for enterprise.
Agent Metadata
Known Gotchas
- ⚠ Test mode API calls look identical to live — easy to accidentally switch to live key and incur real costs and mail
- ⚠ Mail pieces cannot be cancelled once sent to print (usually within seconds of API call)
- ⚠ Custom HTML templates must pass Lob's HTML/CSS validation which is stricter than browser rendering
- ⚠ Delivery tracking via webhooks — pieces don't appear in USPS tracking immediately after API call
- ⚠ International mail support is limited — verify country coverage before relying on for global workflows
Alternatives
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Scores are editorial opinions as of 2026-03-07.