IPhone – iMessage unread count on Mac

iMessage is essentially an Apple operated Instant Messenger service deployed to compete with, or augment, SMS and MMS.

iOS will use iMessage to deliver a message if there is a data connection available to Apple's servers and both the sender and recipient are known by Apple to have iMessage configured for their respective addresses.

Most likely if a person you regularly iMessage with is now sometimes receiving as SMS it is due to intermittent data connection (ie you are mobile and data service is failing). Otherwise it could be an Apple iMessage server issue - seems to crop up regularly.

Furthermore in the last three weeks several of my friends and family have reported issues with iMessage so possibly there is something deeper and worse going on with iMessage servers.

cheers

If the sending phone has no 3G or wifi coverage it will send as an SMS message. Otherwise it gets sent as an SMS. If the receiving phone doesn't acknowledge the message after 30 seconds (because it doesn't have 3G or wifi coverage, or it's turned off) the sending phone will give up on iMessage and send it as an SMS.

If the sending phone has turned off the setting to "Send as SMS" then it won't send as SMS unless the user manually does it.

EDIT: If the receiving user has Messages set up on their Mac running Mountain Lion or Mavericks, and the phone is out of data coverage, the message could be received by the Mac and not the phone.