- Revenue per recipient: what each email earned for every person it went to
- Click rate: how many people did something
- Spam complaint rate: the number that can stop your email reaching anyone
- Unsubscribe rate: how many people decided they had heard enough
- Bounce rate: whether the list is healthy
- Open rate: useful only for comparing subject lines, and not always then
If a term here is unfamiliar, the glossary explains it in a line or two.