Excellent question! What is the texting application you are using? Are you sending a group or single text message?
I can understand your concerns regarding the delivery message. It could mean that the user has delivery conformation turned off, has not read it, or the message was deleted prior to being opened. AlbertoR_VZW
From what I understand, The texts sent that say delivered are sent to Verizon phone customers. Verizon knows that their customer received the text. The texts set to customers of other cell phone providers just show sent. Verizon cannot confirm they received the text. Just that it was successfully sent.