IMessage contacts are not registered

Question

Since two days, when I want to contact some friend(s) on iMessage I got the error message "{contact} isn't registered by iMessage".
On my iPhone there aren't any problems, only on my MacBook Pro 13" non retina 2012.
Is there somebody that can help me? It's really annoying...
Yours sincerely,
Patrick

Answer 1

Hi,
Both FaceTime and Messages (for all accounts) use a support app called IMAgent that does start when the Mac does to "Listen" to the servers from new Calls in the case of FaceTime and new Messages for the Messages app.
IMAgent will not do this for FaceTime if it is specifically turned Off.
In Messages > Preferences > General section there is a a line that reads "When I Quit, Set the Status to Off line".  This refers to all accounts that have Buddy lists  (Bonjour, AIM, Jabber and Yahoo) but as you can see not the iMessages one.
This means you can set everything but the iMessages account to stop Listening to the various servers.
There is also an App called SOAgent that also "Supports" Messages and FaceTime  and has taken over some features that IMAgent used to do.
It seems that part of it's function is to link sites/Accounts that deal with the more social aspects of Facebook, Google+ and others and link them into Messages.
So, whilst not a precise term, things relating to Messages do start up without the app itself when the computer starts up.
To be clear:-
Whilst there does seem like there is a sort of Central Clearing server when you use your Apple ID in various places  it is in fact those server that check and then "register" you and your Computer on that server.
So for iTunes for instance you can link in 5 computers to the iTunes account.  each VM or even separate Install on Partitions and External  drives counts as One in each case  (I have had to "de-register" some older builds that my G4 had over 4 internal drives one of which had a Partition. This includes OS 9 builds).
As the Money in iTunes you might have yet to use also shows up in the App Store there is a closer link here than first appears.
The Game Center  seems to be a separate server (I don't use it so I don't know if the games in use can have In game purchases which would be linked to the app Store).
As the iMessages account in Messages invokes FaceTime to do Video and Audio chats it makes sense these uses the same Apple ID.
The outcome is however you can use different Apple IDs in different places when the overall Apple services.
7:13 pm      Sunday; March 8, 2015
​  iMac 2.5Ghz i5 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