Check your internet connection, tap the red ❗ → Try Again or Send as Text Message, turn on Send as Text Message in Messages settings, make sure the number is correct and not blocked, and toggle iMessage off and on.

Why this happens

iMessages travel over the internet. If you’re offline, iMessage is still activating, or Apple’s servers can’t reach the recipient, the message fails. With Send as Text Message on, iOS falls back to SMS when iMessage isn’t possible.

If only one contact fails, they may have blocked you, changed numbers, or switched to Android without deregistering iMessage.

Signs of the problem

  • Red exclamation mark with “Not Delivered”.
  • Messages stuck sending.
  • Only one contact affected.

Common causes

  • No internet or weak signal.
  • iMessage not activated or signed out.
  • Recipient changed phones/blocked you.
  • Send as Text Message turned off.

Fixes to try, in order

  1. Check connectivityLoad a web page; toggle Wi-Fi or Airplane mode.
  2. Retry or send as SMSTap the red ❗ → Try Again or Send as Text Message.
  3. Enable SMS fallbackSettings → Apps → Messages → turn on Send as Text Message (and RCS Messaging if available).
  4. Verify the contactCheck the number format, and make sure they aren't in Blocked Contacts.
  5. Toggle iMessageTurn iMessage off, restart, turn it back on.
  6. Reset network settingsAs a last resort, Settings → General → Transfer or Reset iPhone → Reset → Reset Network Settings clears saved Wi-Fi, VPN and cellular settings (not your data).
How to confirm the fix: Messages show 'Delivered' under the latest bubble.

Prevent it from coming back

Keep Send as Text Message on so messages fall back to SMS.