According to the Salesforce documentation, the asset that needs to be created first in order for a user to send a prospect a one-to-one email is an email template. A one-to-one email is an email that is sent to an individual prospect, such as a follow-up or a confirmation email. A one-to-one email can be sent from the prospect record, a list, a report, or a campaign. To send a one-to-one email, the user needs to have an email template that defines the content and layout of the email. An email template can be created in Email Studio or Content Builder, and it can be personalized with variable tags or dynamic content. An autoresponder, an engagement studio program email, or a list email are not assets that need to be created first in order for a user to send a prospect a one-to-one email, as they are related to other types of email sends, such as automated responses, program emails, or mass emails. References: Salesforce documentation