If it's just her, and none of your other contacts, and it still works sometimes it definitely is on her end. By any chance is are the message you're sending unsuccessfully media messages? (pictures, videos, etc)?Yeah. That's why I'm so confused it's just texting her sometimes. So it's not like she disabled it.
The last couple of days I've been trying to send texts to a girl and it keeps saying message not sent (error 21) it's driving me crazy, because it's only doing it to her. It sometimes works at night but it doesn't work all day. Please help, thank you.