What does it mean when you send someone a text message and it shows a red check mark next to their message?

[johnnyr787]

The person I sent the message to has a different provider so they are not a Verizon member but for some random people I will send them a text message using my Verizon phone and it shows a red check mark next to the text message I sent them before they even reply. A few of the people that get this red check mark are not even Verizon users. So what is this red check mark and also why do some people I send text messages to get it but others don't?

[tikibar1]

A red check means the text wasn't sent (maybe the number has been changed, they have texts blocked, there was a network problem).