E-nails have a subject line – please use it. The subject should be specific and aid the other person in processing your e-mail.
Also consider who your recipient is. If you are emailing finance then don’t just put ‘Invoice’ in the subject line – but something about more descriptive e.g ‘invoice for front office from Staples (invoice no:3512)
Replying to an old e-mail may be the easiest way of ensuring the mail is correctly addressed, but if you are starting a new topic or discussion ensure you put a new title.