Everything worth knowing, nothing that isn't.
Consent and record-keeping
Collect explicit opt-in for marketing, keep proof of when and how consent was given, and separate transactional messages (which may not need marketing consent) from promotional ones. See /answers/gdpr-for-messaging.
Opt-out and do-not-contact
Process STOP and unsubscribe requests instantly and maintain a suppression list, so a user who opts out never receives another marketing message. See /answers/what-is-dnc.
Sender ID registration and classification
Register alphanumeric sender IDs where required and classify traffic correctly — sending promotional content under a transactional ID can get the ID suspended by the operator firewall. See /answers/how-to-register-sender-id.
Timing, content, and data protection
Respect quiet hours and the recipient's timezone, follow local content rules, and comply with the relevant data-protection law (for example GDPR or a regional PDPL) for any personal data you process. See /answers/pdpl-ksa-explained and the /regulations guides.