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For those who have 26.6 installed and this is working again - how do you have these 3 settings set in Apps/Messages/RCS Messages?

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Nope - does not fix it for me on my Mac. I guess now we wait until 26.6 is out or go to the beta, depending on how serious an issue it is for us. iPhone works for me..
 
Is someone using only an iPhone seeing this issue? RCS stopped working on my phone about a week after I installed the updated iOS 26.5 RC. When I switch the SIM card to an Android phone, it works. I see this for my German numbers. iOS 26.6 Beta 1 did not fix it.
 
RCS also continuously breaks here, forcing me to toggle RCS and reboot my device to get it working for a few minutes again. I still receive RCS texts in group chats though, since my other devices miraculously are still connected to RCS… just my iPhone doesn‘t work.
 
RESOLVED — Possible root cause identified

After several hours of troubleshooting with both T-Mobile and Apple Support, I was able to resolve the Mac send failure issue. Here's what worked and what I believe caused it.

Root cause: A previous logic board replacement on my MacBook caused iCloud to register it as a new device without removing the old entry. This left a duplicate MacBook entry in iCloud Trusted Devices — same name, recent "last seen" date. This appears to have corrupted the local Messages database in a way that allowed incoming messages to work normally while outbound sends failed consistently with "Not Delivered."

I created a new user account on my macbook to clear out any config issues, and things were working there, so I knew there had to be a way to fix my main account.

Fix:

  1. Go to Settings → Apple ID → scroll down and audit every device in your trusted devices list. Remove any duplicates or devices you no longer own.
  2. On the Mac sign out of icloud inside the messages app and quit Messages completely.
  3. Navigate to ~/Library/Messages (in Finder, hold Option → click Go → Library) and delete the Messages folder entirely.
  4. I ran Onyx to clear system and user caches — titanium-software.fr — make sure you match the version to your macOS version. -this may not have been necessary, but I did it anyway.
  5. Restart the Mac! otherwise the system will reload the messages database from the cache which is the problem.
  6. Reopen Messages, sign in and allow iCloud to resync.
Sending from both new and existing threads in my main account is now working normally.

Hope this saves someone an afternoon.
 
It did not work at all.

OS 26.5.x broke everything. Apple can be proud of it, no way.


Just in case, you can install imessage-exporter (free) and backup important SMS messages in a few seconds. Messages will be exported in HTML and it will not be possible to reimport them in Apple Messages, hence at last you will a readable backup of your messages.
 
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