Hi,
Let's see if I have understood this.
You are iMessaging some Contacts from an Mac.
They have Macs and iPhones.
You are concerned the iPhone is retuning the "Delivered" message when you actually want to confirm the receipt on their Mac.
It is best if you use the Apple ID when "Aiming" iMessages as someone's Mac.
Nominally iMessages are Copied at the Servers end and delivered Uniquely to each device.
Having said that it means the "original" goes to the device that is the lead for that ID (or iPhone Number) and that device returns the "Delivered" or "Read" subtext messages.
Therefore iPhone will "respond" to iPhone Numbered iMessages whereas the Mac will respond to Apple ID ones.
MORE...
This is further complicated by the settings made in Send and Receive on the iPhone with the "Start Conversations From" and the similar setting on the Mac.
This can mean that the Mac is "spoofing" the iPhone Number in effect but it sort of raises the priority of the iPhone Number on the Mac for the receipt of iMessages as well as Starting new conversations.
You can set each device to have it's Own ID as the "Start Conversations From".
On my iPhone it is set as the Number and on the Mac as the Apple ID.
Rather than setting a Block (which gets synced across all devices) you can turn Off the Apple ID on the iPhone (See Send and Receive) and the iPhone Number on the Mac.
This way the individual Device will be the one responding.
Whilst this works for short periods (and keeps both devices registered) prelonged setting it this way can Unlink the two devices.
Of course if you don't know the Apple ID you are going to be stuck anyway.
9:04 p.m. Tuesday; August 25, 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