Open your draft. Read the subject line aloud, in the voice you would use leaving someone a voicemail.

Most subject lines fail instantly. Nobody has ever rung a colleague and said "unlocking synergies in Q4". Nobody has said "your complete guide to" anything. You hear it and you wince.

The ones that pass sound like a person with a reason to be in touch. "The thing we talked about on Thursday." "Bad news about the deadline." "Three numbers from the pilot."

This works because email is a one to one medium wearing a broadcast costume. Your reader is not an audience. They are one person looking at a list of names and sentences, deciding what deserves a click.

The test takes four seconds and it will kill about half of what you write. That is the point.

If you would not say it on the phone, do not put it in the inbox.