What means a red triangular on top of the envelope icon and how i can fix that

[AC Question]

It is a triangular icon on red on top of the envelope when i close the text maseege of the person i just sent but when i open the text maseege of the person doesn't show any is only on the notification page

 

[B. Diddy]

Welcome to Android Central! Which phone? Are you saying this icon is in the notification bar/panel? It probably means there was a problem sending the message. Do you know if the recipient actually got the message?