Send16

List messages from the unified send log (transactional + campaign). Filter by recipient, sender, status, date range, or source.

List dynamic segments. Segments compute their members from a filter expression at send time.

List suppressed email addresses (bounces, complaints, unsubscribes, manual).

List starter-gallery email templates available to every workspace.

List subscription topics. Topics are granular categories contacts can subscribe to (e.g. 'product-updates', 'weekly-digest'). Campaigns and automations can be gated to subscribers of a specific topic.

List configured outgoing webhooks (your endpoints that Send16 POSTs delivery events to). Outgoing webhooks are a plan feature.

List the templates saved in the user's own Send16 workspace (their template library, including ones saved with save_template). For the shared starter gallery use list_templates.

List workspaces the authenticated user has access to (owner or member).

Mark a conversation as read for the current viewer.

Pause an automation. Existing enrollments freeze in place; no new contacts enroll.

Pause a sending campaign. Already-sent messages are unaffected; remaining sends halt.

Run an AI pre-send review on an email's HTML before sending. Surfaces tone issues, broken links, missing alt text, spam triggers, and accessibility problems.

Preview the recipient set for a campaign before sending. Returns count + sample list.

Preview the contacts that match a segment without sending.

Rotate the HMAC signing secret for an outgoing webhook endpoint. Returns the new secret once.

Register someone for an event. They get the event's confirmation email with their QR ticket unless the organiser switched it off, so confirm the person and the event with the user first. Registering doesn't subscribe them to marketing unless marketingOptIn is true because they asked.

Remove a contact from an audience.

Remove an email from the suppression list. Use only when you've confirmed the recipient wants to receive mail again.

Compile an EmailContent JSON object (the shape used by the Send16 builder + templates) into HTML using the platform's authoritative renderer. Returns the exact HTML the API would send. Useful for previews and post-processing.

Resume a paused campaign. Confirm with the user first: the remaining recipients start receiving the campaign straight away. The sending limits and pauses described on schedule_campaign still apply.

Save an email design as a reusable template in the user's Send16 workspace. Use this when the user has designed an email (for example in a Claude artifact) and wants it in Send16. Pass the complete email HTML. Send16 hosts every image (remote URLs and inline data: images), converts merge tags like {{first_name}} to Send16's format, and stores the HTML exactly as designed. For best results across Gmail, Outlook and Apple Mail the HTML should be email-safe: table-based layout around 600px wide, inline styles, PNG/JPG/GIF images in <img> tags (not SVG or CSS backgrounds), web-safe font fallbacks, and alt text on images. Returns the template id; follow up with send_template_test to send a preview.

Schedule (or send immediately) an existing draft campaign to an audience/segment/topic. Before calling, confirm with the user: show them the recipients (or the recipient count), the sender and the subject, and wait for a clear yes. Send16 can refuse a send. 429 SEND_RAMP_LIMIT: new and free workspaces have hourly and daily sending limits while they build reputation; the error details give the window, the limit and how many remain. Tell the user how many can go now and that the rest can go once the window resets (or after upgrading), and do not split the send or retry in a loop to get around it. 403 SENDING_PAUSED: sending from the workspace is paused while the Send16 team reviews it. Nothing was sent. Tell the user to check the workspace owner's email or contact support, and do not retry. 422 MESSAGE_NEEDS_REVIEW: the content looked like it might be impersonating a government agency, bank, toll operator or delivery company, so it was held and not sent. Tell the user it is held for review and they can contact support if it is legitimate. Do not reword it and try again; repeated flags pause the workspace.

Decide a pending screener conversation (first-time sender). 'allow' lets future emails through, 'reject' blocks, 'cold' marks as cold outreach.

Search contacts by email address or name. Returns matching contacts with their tags and subscription status.

One-shot broadcast: create a campaign and immediately schedule it to send, in a single call. Pass either an audienceId, segmentId, topicId, or fully-qualified filter. Provide html OR templateSlug. Before calling, confirm with the user: show them the recipients (or the recipient count), the sender and the subject, and wait for a clear yes. Send16 can refuse a send. 429 SEND_RAMP_LIMIT: new and free workspaces have hourly and daily sending limits while they build reputation; the error details give the window, the limit and how many remain. Tell the user how many can go now and that the rest can go once the window resets (or after upgrading), and do not split the send or retry in a loop to get around it. 403 SENDING_PAUSED: sending from the workspace is paused while the Send16 team reviews it. Nothing was sent. Tell the user to check the workspace owner's email or contact support, and do not retry. 422 MESSAGE_NEEDS_REVIEW: the content looked like it might be impersonating a government agency, bank, toll operator or delivery company, so it was held and not sent. Tell the user it is held for review and they can contact support if it is legitimate. Do not reword it and try again; repeated flags pause the workspace.

Send a test email for an existing campaign to preview how it will look in recipients' inboxes. Confirm the test address with the user first. Can be refused with 403 SENDING_PAUSED (workspace under review, nothing was sent, tell the user to contact support).

Send a transactional email through Send16. Provide `html`, `text`, or both (text-only is fine for OTPs/receipts/alerts). `from_email` must belong to a domain that is verified in the workspace; call `whoami` for the account and `list_domains` to see which senders are allowed; a send from an unverified domain returns a clear error, not a silent failure. For a zero-setup first send, use from_email `onboarding@send16.com`; the sandbox sender delivers with no DNS, but only to your own account email. Before calling, confirm with the user: show them the recipients (or the recipient count), the sender and the subject, and wait for a clear yes. Send16 can refuse a send. 429 SEND_RAMP_LIMIT: new and free workspaces have hourly and daily sending limits while they build reputation; the error details give the window, the limit and how many remain. Tell the user how many can go now and that the rest can go once the window resets (or after upgrading), and do not split the send or retry in a loop to get around it. 403 SENDING_PAUSED: sending from the workspace is paused while the Send16 team reviews it. Nothing was sent. Tell the user to check the workspace owner's email or contact support, and do not retry. 422 MESSAGE_NEEDS_REVIEW: the content looked like it might be impersonating a government agency, bank, toll operator or delivery company, so it was held and not sent. Tell the user it is held for review and they can contact support if it is legitimate. Do not reword it and try again; repeated flags pause the workspace.

Fire a custom event for a contact. Automations whose trigger is `{type:'custom_event', eventName:'<name>'}` will enroll the contact and start running. Payload is exposed to condition steps as `event.<key>`. Confirm with the user first if the event could start an automation that emails the contact.

Reply to an inbox conversation. The reply is sent from the original recipient address (the alias the convo arrived on) and threaded back to the contact. Show the user the reply text and get a clear yes first: it goes to a real person and cannot be unsent. Can be refused with 403 SENDING_PAUSED (workspace under review, nothing was sent, tell the user to contact support).

Send a preview of a saved workspace template to one email address, so the user can see it in a real inbox. It arrives from noreply@send16.com with a small 'Template test preview' banner and uses sample merge values (first name 'Test'). Only send to the user's own address unless they ask otherwise. Confirm the test address with the user first. Can be refused with 403 SENDING_PAUSED (workspace under review, nothing was sent, tell the user to contact support).

Update an Action (toggle enabled, change template, change event name).

Update workspace brand settings. The designSpec field is a Markdown DESIGN.md document fed into AI generation system prompts; use it to encode voice, do/don't lists, and visual rules. companyName/websiteUrl/companyAddress also fill the matching merge tags in templates.

Update a draft campaign: name, subject, preview text, sender info, HTML content, and/or recipients. Retarget with exactly ONE of audienceIds / segmentId / contactIds / tagIds / sendToAll, then verify with preview_campaign_recipients before sending. Only drafts can be edited.

Update a contact's profile fields, custom fields, or subscription state.

Update conversation status (open/closed/archived/snoozed) or assignee.

Update a subscription topic (name, description, default).

Verify the API key and return the authenticated workspace: name, plan, email quota (used/limit), and owner. Call this first to confirm the key works before sending. Pair with list_domains to see which `from` addresses are allowed.