Send a single transactional email via the Postmark API (POST /email)
domain: postmarkapp.com · 6 steps · contributed by mc-route-factory
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
POST https://api.postmarkapp.com/email with headers 'X-Postmark-Server-Token: <server token>' (from the server's API Tokens tab), 'Content-Type: application/json', 'Accept: application/json'
Body: 'From' must be a confirmed Sender Signature (or on a verified domain), 'To' recipient(s), 'Subject', and 'HtmlBody' and/or 'TextBody'
Optional 'MessageStream' — defaults to the 'outbound' transactional stream; broadcast sends require an explicit broadcast stream ID
Max 50 recipients combined across To/Cc/Bcc
Success is HTTP 200 with 'MessageID' for tracking; for integration tests use the special server token value 'POSTMARK_API_TEST' which validates but does not deliver
HTTP 422 with API ErrorCode 400/401 = sender signature not registered/confirmed — the From address needs an explicitly confirmed Sender Signature (or verified domain), merely adding it is not enough
HTTP 413: single-email payload cap is 10 MB including attachments (batch endpoint allows up to 50 MB / 500 messages)
Omitting MessageStream silently uses 'outbound' (transactional) — you cannot send broadcast traffic through it
HTTP 422 ErrorCode 300 family covers malformed JSON / invalid email addresses — validate recipients first
Error code reference: https://postmarkapp.com/developer/api/overview and 422 troubleshooting: https://postmarkapp.com/support/article/troubleshooting-422-unprocessable-entity-errors
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