What does it mean to receive a short message without a theme?

I received an untitled email for the following reasons.

1. When the other party sends a short message, it does not add a short message subject or adds the wrong short message subject. The message sent by the other party is spam or harassing text messages, deliberately not showing the theme.

2. The short message sent by the other party failed to add a theme or failed to add a theme, which led to the failure of sending.