If iPhone is not Sending iMessages, you should be able to fix the problem using troubleshooting steps as provided below.
iPhone Not Sending iMessages
When you send Messages from iPhone, Apple makes use of its own iCloud Network to deliver the Messages in free iMessage format to all Apple Device Users.
If some of your Contacts are not using iPhone, the Messages App makes use of your Carriers network to send the Message in the form of a paid SMS Text Message.
Hence, the issue of iPhone not sending iMessages is mostly related to a problem with Apple’s iMessage Service or due to your Contact switching from iPhone to Android Platform.
Apart from this, the problem could be due to network related issues, change in network settings, incorrect iMessage settings and other reasons.
1. Check Status of Apple’s iMessage Service
Apple’s iMessage service might be temporarily down or experiencing problems. Hence, it is a good idea to rule out this possibility by going to Apple’s System Status page.
On this page, Apple provides a list of all its services and their current status of service (Available or Unavailable).
2. Make Sure iMessage is Enabled
Sometimes, the problem is due to Apple’s iMessage service being accidentally disabled on your device.
Go to Settings > Messages > on the next screen, make sure iMessage is enabled and there is no error message on this screen.
As you can see in above image, iPhone is unable to send iMessages due iMessage Activation error.
If you are also seeing a similar error message, you should be able to fix the problem using steps provided in this guide: Fix: iMessage Waiting For Activation Error on iPhone.
3. Check Network Connectivity
Go to Settings > WiFi > make sure iPhone is connected to WiFi Network.
Similarly, go to Settings > Cellular and make sure Cellular Data is enabled.
After this, try to send the iMessage again and see if you are still having problems.
4. Set Date & Time Automatically
It is quite possible that your iPhone is Not sending iMessages due to “Date and Time” settings being incorrect.
Go to Settings > General > Date & Time > On the next screen, make sure Set Automatically is Enabled and iPhone is showing the correct Time Zone.
6. Check Send/Receive Settings
A common reason for iMessage problems on iPhone is due to incorrect Send/Receive settings.
Go to Settings > Messages > scroll down and tap on Send & Receive.
On the next screen, make sure your Apple ID Email Address is listed under “You Can Be Reached By iMessage At” section of iMessage screen.
Since, iMessage makes use of Apple ID to send and receive iMessages, your Apple ID needs to be listed on this screen, in addition to your Phone Number.
7. Sign-in/Sign-out of Apple ID
iMessage problems can be caused due to your Apple ID not being recognized by Apple Servers.
Open Settings and tap on your Apple ID.
On Apple ID screen, scroll down to bottom and tap on the Sign Out option.
On the next screen, tap on the Sign out link located at the top right corner of your screen.
On the confirmation pop-up, tap on Sign out to confirm. After you are completey signed out, tap on Sign-in to your iPhone link.
On the next screen, enter your Apple ID and Passcode to sign-in to your iPhone
This should fix the problem, if it was due to your Apple ID not being recognized by Apple servers.
8. Delete Old iMessages
Sometimes, the problem of iMessages not working on iPhone is caused by an older iMessage getting stuck in an iMessage loop.
Some users have reported fixing this issue by deleting the Last iMessage received prior to the start of the problem.
Others have reported fixing this problem by deleting the entire iMessage thread and starting a New iMessage thread.