Remember that e-mail is just another form of memo with its special protocol. The subject line either makes or breaks the memo. The subject line is usually expressed in about three-five words. It should carry the impact of what the memo is about. The subject line often becomes the most neglected part of the memo.
People assume they know what the subject is.
If you are writing a memo about a listening experience you had, you assume the subject is what was talked about. That does not follow. The subject line tells what the memo is about, not what topic of listening occurred. That format immediately suggests a memo without your having to tell the reader again. Also, you have to be careful of the political significance of the subject line.