iMessage not working on iMac with El Capitan

Log out of iMessage on the iMac and restart. Log back in and see what happens.

HI,

Try these things:-

1) In the Apple Icon Menu top left use the About this Mac option.

In the Overview tab the Serial Number should be visible.

If it is not then visit an Apple Store or Apple Authorise Service Provider (AASP) to have it put back.

It is often missed when repairs to the Logic/Mother board are done.

2) Rest the NVRAM How to Reset NVRAM on your Mac - Apple Support

3) Boot in Safe Mode and them try Messages.

Whether it works or does not reboot normally.

Since OS X 10.8.2 the Messages app has used the Serial Number as part of the Login.

It is part of the process that make s logins device specific.

This means that the iMessages server can lock out the use of the Apple ID on just one device.

Most often this is the Mac.

4) Try logging into a another Mac User Account and setting up Messages with your details.

If this works it is your Users account that has a problem.

If it does not work it is the iMessages Servers.

5) Contact Apple Support by phone.

State you have an Apple ID issue (Free to to fix)

Level one responders are script led and trying to fix Apple Care issues with Software and Hardware.

You are likely to have to ask to speak to a Level 2 or higher assistant.

They still may want to do data gathering and trying some other things.

You do NOT need to Update if at a slightly older version of the current OS.

you do NOT need to Upgrade if using an earlier OS.

You do NOT need to Reinstall.

Insist they check the server and they should be able to see that it is your Mac using the Apple ID that is blocked.

It is likely at this point they will involve the Engineers who will eventually reset the server but you are likely to be told it will take 24 hours.

We have not found a quicker way to achieve this.

9:30 pm Friday; May 6, 2016

Same here. Cannot find a fix. Appears to be faulty Apple product - in the back end.

I'm using 2FA and wondering if that is the problem.

Hi,

If you are using 2 Step Verification then you do have to create an Application specific password for Messages.

8:25 pm Monday; May 23, 2016

DDid this on two different computers. Created multiple app specific password, None can

logo.

STill broken. Mega-annoyed

Hi,

I found this if you had not already seen it.

Using app-specific passwords - Apple Support

There is an issue for some at the server end.

The Mac login needs the Apple ID and password but also the Serial Number so that the Login is device specific.

If a NVRAM rest has not worked

Nor a Safe Boot and

the Serial Number can be seen in the About this Mac info

Then it is likely to be time to call Apple Support.

The Level 1 people who answer are script led.

You need to explain that you have an Apple ID issue (Outside their script). These are free to fix.

You may have to ask to be moved up the chain.

Even then they are likely to want to data gather and try s few things.

Reinstalling, Updating or Upgrading will not work.

Insist that they reset the iMessages server just for your Mac's use of the Apple ID.

They are likely to involve the Engineers and tell you it will take 24 hours.

From others who have posted back about this it normally works overnight.

7:21 pm Tuesday; May 24, 2016

On your Mac go to the (imessage app). In the upper left corner click the bold letters that say (messages) then click (preferences) afterwards you will see a blue @ icon that says (accounts) from there you will see bonjour, gmail, yahoo, etc. you are loooking for the (imessage icon) click it, make sure you are signed in your apple ID and it should cooperate from there. (Let me know if it works) cao

hi,

On your Mac go to the (iMessage app).

It is the Messages App.

iMessages is just one of five different account types it can use (iMessages, Bonjour, AIM, Jabber which includes Google and Yahoo).

On the basis that the Original Poster said:-

... contacts are hi-lighted in red, ...

Then the account has an Apple ID in the Apple ID space in the iMessages account.

From there the issue is how well that account is logged in.

All Contacts going red means the app is not getting feedback from the iMessages Server about who of your Contacts is On or Off Line (it is effectively lying that they all are but using a standard 'nil Response' error message to tell you).

Also the Original Poster has never been back to say one way or another if the problem was resolved.

8:24 pm Thursday; August 25, 2016