I am sending an iMessage to a relative direclty to their apple id (email address). They receive it via their iPad, but not their iPhone, although they have that same apple id (email address) added as a "receive at" on their iPhone. They have their caller id set as that apple id also.
If they have the same email address used for both the iPad and iPhone then the servers will probably send it to the one it 'finds' first or perhaps it uses some other mechanism to decide which of the two to use.
I have 4 iDevices--an iPhone 4, iPhone 4S, and two iPad2's. I get around the problem by setting up a separate email address for each device and register that email address for each device in iMessage (also FaceTime). I have the email addresses for each in my Contacts list.
When I send to the iPhones I use the phone number though I could also use the email address assigned. One has to add a new email address that can be verified or one can't delete the Apple ID email address which is automatically assigned.
When sending to the iPads I use the email address I registered for the destination iPad.
It all works very well, but does require more than one email address (NOT more Apple IDs, I do this all under one Apple ID).
In the table below email address #1 is the email address associated with my Apple ID. It is not my iCloud email address. I could easily have used my iCloud email address for one of the units but I chose not to. I felt it was more flexible to me to use an email address I have control over.
IDevice iMessage address
iPhone 4S Phone number or email address #1
iPhone 4 Phone number or email address #2
iPad #1 email address #3
iPad #2 email address #4
My email provider allows unlimited email aliases. With the exception of #2, which is my wife's email address, the above email addresses all resolve to email address #1. FaceTime and iMessage don't care. As long the email address associated with a device is unique and can be verified, i.e. it must receive a verification email containing a link that can be clicked on, iMessage and FaceTime will connect that iDevice's particulars with the verified email address.
People trying to send iMessages or to FaceTime with one of my two iPads needs to know the email address associated with each iPad. All they need for the iPhones is the phone number.
i have an iphone 3g 3gs 4 and 4s also an ipad 1 and 2
what i do is i use my email (apple id) registered for both my iphone 4s n ipad 2
my husband has his iphone 4 and ipad 1 registered with his he uses his number and email
both my kids use the other iphones and just send to our number
PS if you send it to an email address its more likely it will get emailed
PSS if they are recieving the message in one and not both then they either used a different email (apple id)
Hopes this helps