"Your message could not be sent. xxx@xxx.com is not registered with iMessage."

HI,

So this is Messages with an iMessages Account and at least one Group chat to iMessages contacts.

Sometimes some Messages go red after a delay.

Each time you are the person sending two iMessages one after the other you get the Alert.

The XXX.@xxx.com would seem to lean towards the Contact using an older @Mac.com Apple ID

Is this the case or is it just the style you used ?

Can you also tell me if this is the "first" person in the group ?
As the Last person in the group that sent you something is the "current" "front" Buddy pic in the List on the left does the error message list their Apple ID ? (As in being different from my other suggestion).

If you have not already try this.

Close/Quit Messages.

In Applications/Utilities open Activity Monitor

Find the imagent item (Lower case and no icon)

Highlight and use the Quit Item top left and Confirm Force Quit.

Restart Messages.

10:31 pm Saturday; August 10, 2013

 iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images."  No, Seriously

Hi,

Does the things you have tried include Signing Out in Messages > Preferences > Accounts > iMessages account and then quitting the App and re-entering the Apple ID after restarting ?

If this Apple ID is linked to an iPhone Number on an iPhone does the iPhone show both the iPhone Number AND Apple ID as Received At and Send From IDs ?

10:23 PM Monday; August 12, 2013

 iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images."  No, Seriously

Hi,

If you are using Messages in Mountain Lion can you use the Messages Menu > Accounts.

This should list all the account actually in Messages Menu > Preferences > Accounts with ticks against the ones that are logged in.

The ones that that then have Buddy lists will appear in the Window Menu (Possibly as the Combined "Buddies" list or as separate ones)

If iMessages is not active then it will not be ticked in the Messages > Accounts list.

This List is not in the Messages 8 version in Mavericks.

In either case in Message Menu > Preferences > Accounts > iMessages account does the "Enable this Account" show as ticked ?

Have you tried Unticking it and re-ticking to log it in again ?

Is the Firewall in System Preferences > Security and Privacy > Firewall Tab, On ?

If so use the Options button and see if the "Allow Signed Software" is Enabled.

Is the Messages app also in the list if this method is used ?

7:39 pm Sunday; January 26, 2014

 iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad

You are welcome.

Thanks for letting us know.

8:00 pm Thursday; February 27, 2014

 iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
 G4/1GhzDual MDD (Leopard 10.5.8)
 MacBookPro 2Gb (Snow Leopard 10.6.8)
 Mac OS X (10.6.8),
 Couple of iPhones and an iPad

You're welcome

7:53 pm Thursday; November 17, 2016

hi

am trying to log in to i message using my i mac but i faced this mesage

Your message could not be sent.

Your iMessage account is not signed in.

FYI am already logged in but when i tried to send something it's give it to me this message ,tried to used diffrent apple ID but it is not work with me

regards

"Close/Quit Messages.

In Applications/Utilities open Activity Monitor

Find the imagent item (Lower case and no icon)

Highlight and use the Quit Item top left and Confirm Force Quit."

This works for me.

I've got the same problem as ggoodman1022's and it has been annoying me for weeks until I try your suggestion.

Thank you very much. 🙂

I had this exact same problem just now (trying to use Messages to send an SMS to a friend, with this exact error message).

Did a quick copy-paste of the error into Google, and your post was the first thing that came up.

Tried this bit that you posted, and it worked PERFECTLY:

----------

If you have not already try this.

Close/Quit Messages.

In Applications/Utilities open Activity Monitor

Find the imagent item (Lower case and no icon)

Highlight and use the Quit Item top left and Confirm Force Quit.

Restart Messages.

----------

So, thanks for the help Ralph! Over 3 years after-the-fact, and your post is still helping others. 😀