When sending an email you should summarize the message in the subject line. It will make identifying and prioritizing emails much simpler for both you and the recipient. If, in an email thread, you change the topic, change the subject line to match. Your co-workers will be wildly appreciative.
Think of the difference between a subject line that says,
“Rehearsal on Saturday at 4:00 at the Clubhouse, everyone please confirm.”
Which email do you think will be read and acted upon?