Why is there a horizontal line under the other party's text message?

It was not cancelled when it was set before.

The other party has given you permission (you are not allowed to view his circle of friends), so it is normal for you to send messages to the other party. The other party has added you to the blacklist (in this case, you send him a message indicating that the other party refuses to receive your message.

The other party has deleted you from the address book, and sending him a message will prompt that "the other party has started friend verification, and you are not his friend yet".