Documented steps

POST https://api.postmarkapp.com/email/withTemplate with 'X-Postmark-Server-Token' header (same auth as /email)
Body: 'From' (confirmed sender signature), 'To', and exactly one of 'TemplateId' (integer) or 'TemplateAlias' (string; starts with a letter, only letters/numbers/./-/_)
'TemplateModel' object supplies the variables the template references, e.g. {"user_name":"Jo","order_id":123}; the template defines Subject/HtmlBody/TextBody
Optional 'MessageStream' as with plain sends; response is HTTP 200 with 'MessageID'
Docs: https://postmarkapp.com/developer/api/templates-api

Known gotchas

TemplateId and TemplateAlias are mutually exclusive — sending both (or neither) is a 422 validation error
ErrorCode 1101 = template not found; check the numeric ID or alias spelling/character rules
A TemplateModel missing variables the template references causes 422 template errors (110x range) — send every referenced key
Do not send 'Subject' expecting it to override the template; the template's subject wins unless the template itself uses a variable for it
Same sender-signature rule as /email: unconfirmed From → 422 ErrorCode 400

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Send an email with a Postmark template (POST /email/withTemplate)

Documented steps

Known gotchas

Related routes

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