I want to be certain of the recipient's identity (and vice versa)
When sending an email, it is incredibly easy to fake (or “spoof”) one’s email address. For example, try sending yourself an email with a made up identity from https://emkei.cz
In order to be certain of each other’s email identity the message needs to be signed cryptographically. You can do this yourself or learn about existing authentication methods used by some email providers.