Hello Eewar_,
Welcome to Apple Support Communities. We understand that when trying to send text messages to an Android device you receive an alert that says" Not Delivered." However, the recipient is actually receiving the message. We'd like to provide you with some steps that may help resolve the issue:
• Restart your iPhone
• Manually update your carrier settings on your iPhone or iPad
• Reset your Network Settings. If you're using iOS or iPadOS 15 or later, tap Settings > General > Transfer or Reset [Device] > Reset > Reset Network Settings. If you're using iOS or iPadOS 14 or earlier, tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
• Update iOS on iPhone: Go to Settings > General > Software Update to verify if you're running the latest software version. (Secure your data with a backup before completing this step)
If you continue to experience the same behavior, the issue may be related to your network. Reach out to your carrier for further assistance.
Best wishes!