Send
Sending email
Send transactional messages, batches, replies, attachments, and hosted templates safely.
Message content
Provide a from identity on a verified workspace domain, at least one to recipient, and HTML, text, or one published hosted template. A raw-content send may omit subject, which produces an empty subject. SendHQ generates a text alternative from HTML when needed, but an authored alternative is more reliable.
Recipients and credits
A message may contain at most 100 combined To, Cc, and Bcc destinations. Each destination consumes one recipient-delivery credit. Duplicate normalized recipient addresses are checked once for suppressions, while every destination still counts toward message limits.
Safe retries
Send a stable Idempotency-Key of at most 200 characters on single and batch submissions. Reusing a key with the identical JSON payload replays the stored response and adds Idempotent-Replayed: true. Changing any recipient, content, header, or template data under the same key returns 409; a concurrent request with that key also returns 409 while the first is pending.
Batch sending
POST /emails/batch accepts up to 100 message objects and returns a result for every position. Use separate message objects for personalized template data rather than sending one personalized template to multiple recipients.
Replies and threads
Set reply_to_email_id when replying. SendHQ derives Message-ID, In-Reply-To, References, and the existing thread ID. Retrieve the complete conversation with GET /threads/:id.