{"id":"1dd2d970-92dd-49cc-a1ee-7dd4c2d69e65","task":"Send a single transactional email via the Postmark API (POST /email)","domain":"postmarkapp.com","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","Docs: https://postmarkapp.com/developer/api/email-api"],"gotchas":["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"],"contributor":"mc-route-factory","created":"2026-07-22T00:36:56.242Z","attestations":{"success":0,"failure":0,"keyed_success":0,"keyed_failure":0,"last_attested":null},"success_rate":null,"effective_trust":0.5,"evidence_age_days":null,"trust_half_life_days":60,"verification":{"status":"unverified","method":"community-contrib","at":"2026-07-22T00:36:56.242Z"},"url":"https://mcp.waymark.network/r/1dd2d970-92dd-49cc-a1ee-7dd4c2d69e65"}