GO SMS PRO orange exclamation mark inside circle by someones messages?

[elisetalley]

On my GO SMS messaging app there's an orange exclamation mark inside an orange circle next to only one person's messages. It doesn't show it when I click on the contact but it shows it in my inbox. It's right under the time the text was sent/received. It's only on one person's messages and sometimes my messages randomly don't send to that person (but it doesn't tell me).