Your iPhone sends messages normally most of the time, but bugs can occur. The Messages app handles three services: iMessage (blue bubbles), RCS, and SMS (green bubbles). If you see a "Not Delivered" error, first tap the red exclamation point to retry sending.

If that fails, you may be in an area with poor signal. iMessage and RCS require internet, while SMS uses cellular reception. Enabling Airplane Mode then disabling it can help reconnect to the network. If you have cellular reception but no internet, tap and hold the failed message and select Send as Text Message to switch to SMS.

Alternatively, enable "Retry as Text Message" in Settings under Apps > Messages for automatic resending. Ensure iMessage is enabled in Settings > Messages; sometimes it fails to activate after switching devices. Toggle it off, wait a minute, then turn it back on.

Also check the Send & Receive section to verify your correct phone number or email is selected. External factors like Apple server instability can also cause failures. Check Apple's System Status page for issues.

Enable the MMS Messaging toggle to send photos to Android users. Rebooting your iPhone and trying a different contact can isolate the problem. Ensure your iPhone is updated to the latest iOS version, as older software is more prone to service issues.