Message Fail to Send Error on Restart

Good day. Every time I restart my iPhone 6, within a minute I receive a notification saying, "The message to Jane Smith failed to send." (Contact name has been changed) It's been doing this for 715 days, according to the latest notification. I've tried deleting the message thread, deleting and rebuilding the contact, and it's still doing it after I installed iOS 12. This contact receives text messages, not iMessages, if that's any help. More an annoyance than anything else, but any help would be appreciated. Thank you.