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Two things protect your recipients and your sender reputation: the automatic opt-out that always applies, and suppression lists you control. On top of those, every send observes quiet hours and frequency limits.

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Opt-outs are automatic

A contact who replies STOP (or a standard opt-out keyword) is opted out immediately and never messaged again — across every campaign and flow. You don’t manage this by hand, and no configuration can override it. HELP and other standard keywords are handled too.
Deleting a contact is not an opt-out. A deleted contact you later re-import can be messaged again. To stop contacting a number permanently, put it on a suppression list.

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Suppression lists

A suppression list is a set of phone numbers to keep off your sends for your own reasons — do-not-contact requests, litigators, competitors, prior customers.
1

Create a list and add numbers

Create a suppression list and add numbers (E.164). You can keep several — e.g. a global DNC and a per-client list.
2

Attach it to a campaign

In the campaign builder’s Audience step, tick the suppression lists to apply. Any recipient whose number is on a checked list is skipped — on top of the automatic opt-out.
3

Manage over time

Add or remove numbers as needed; removing a number lets campaigns that reference the list message it again.

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Quiet hours & frequency

  • Quiet hours — sends respect a local-time window (the TCPA 8am–9pm floor is the recipient’s local time). Where a contact’s timezone is known it’s used; otherwise your workspace policy applies. Configure your window in workspace settings.
  • Frequency limits — optionally cap how often a contact is messaged (a minimum gap between sends and a maximum per rolling window), counted across all campaigns and flows so no single track can over-message someone.
Both default to off until you set a window/limit, and both withhold messages rather than dropping them — a contact outside their window is sent later, not skipped.

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US A2P (10DLC)

Sending A2P SMS in the US requires brand and campaign registration with the carriers (10DLC). Register your brand, then your messaging use-case (campaign), before you send at volume. See the 10DLC endpoints under API reference; registration is distinct from your sending campaigns.

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Over the API & MCP

Manage suppression lists programmatically — Suppression — or in natural language via the MCP server.