Three keywords are non-negotiable in US SMS marketing: STOP, HELP, and START. They're mandated by the TCPA, by carrier rules, and by the CTIA Messaging Principles & Best Practices. If your platform doesn't handle them automatically, you have a problem you may not have noticed yet.
STOP — and its cousins
When a recipient texts STOP to your number, you must (a) send one final confirmation message, (b) immediately stop sending them marketing messages, and (c) maintain that opt-out indefinitely. The TCPA-recognized variants you also need to handle: STOPALL, UNSUBSCRIBE, CANCEL, END, QUIT, OPTOUT, and OPT-OUT.
Treating STOP as a “preference” instead of a hard suppression has cost companies seven-figure TCPA settlements.
Real-life gotcha: opt-outs are scoped to your sender (the long code or short code). If a recipient stops one of your numbers and you switch them to a different number for a different campaign, that's a TCPA violation. Treply enforces a global per-organization suppression list across every sender by default. Most platforms don't.
HELP
HELP requires you to send a message back identifying your brand, what the recipient's subscribed to, support contact info, and instructions to opt out. Skipping this is the easiest way to fail a CTIA short-code audit.
START / UNSTOP / YES
These re-subscribe a recipient who previously opted out. Note: you may not solicit re-subscription from someone who's opted out — the recipient has to initiate. If you send “Reply YES to come back!” to a STOPped recipient, that send itself is the violation.
What Treply does for you
- Auto-responds to all standard variants (configurable language).
- Hard-suppresses globally on STOP, with optional per-channel scoping you have to deliberately turn on.
- Logs every keyword event with timestamp + sender + recipient — your audit trail.
- Surfaces “STOPped” status in your contacts table, audience builder, and campaign send preview.
If you're evaluating an SMS platform: ask them to show you the STOP suppression UI. If they have to schedule a follow-up to find it, keep looking.