Unconfirmed Messages

I have six unconfirmed messages for which I have not received a reply. Does this mean that the individuals would not confirm my message, did not receive them or their mailbox is too full and they haven’t read them yet? Is anyone else having this problem?

If their mailbox was full and unable to receive the message, you would have received a message indicating that when you tried to send it and it wouldn't have went through. There's no way to tell the difference between a message that has been read or not, if the person didn't click "ok" on the read receipt button.

some times it doesn't give me the option to confirm the read receipt if I check messages from the mobile mailbox. Could be that they are using it as well. Or they may have just clicked no or cancel or whatever the option is other then yes.