strange mail behavior

Hi

I have script to to take backup and send mail to a group once a day.
One strange behavior I have observed recently is that most of the time the mail we receive is fine . But someday it just sends out mail without any subject with undisclosed recipients. I dont know how to find the cause her. Syntax I am using is :

echo "Some text ...." | mail -s "$HOSTNAME some text..." mail_id

Please let me know if anyone has faced the same before...