Create another chat group in the Messages and see if this chat group has the same error as the old chat group. If yes, go to Settings> General> Reset> Reset Network Settings .
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Also you can fix this error by backing up and restoring iMessages data:
Go to Settings> iCloud> Storage & Backup> Back Up Now . If you haven't backed up data via iCloud ever, you need to make sure that the iCloud backup feature is turned on.
After you have backed up the data, delete all content and settings by going to Settings> General> Reset (Reset)> Reset All Content and Settings (Delete all content) and reinstall) . After installation is complete, choose to restore data from backup.
5. The error of iMessage failed to send the message
Another error of iMessage is not to enter and send messages. This is because you installed heterogeneous iMessage through email and phone number. If this is the case, do the following:
Go to Settings> Messages> Send & Receive . Then remove the checkmark in front of all email addresses.
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Go back to the Messages section, then turn off iMessages .
Go back to Settings and go to Facetime . Continue to uncheck all email addresses here and turn off Facetime.
Wait about 5 minutes then turn on iMessage and Facetime again . Now you will see that there are no more email addresses here and instead your phone number has been stored in the front.
Go to iMessages and send messages as usual.
6. Error iMessage showing email address instead of your phone number
Go to Settings> Messages> Send & Receive> Send & receive> Apple ID> Sign Out . iMessage will now display phone numbers instead of email addresses.
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