A Marketing Manager wants to send out an email to a list of prospects that...

According to the [Salesforce documentation], the best way to accomplish the scenario of sending out an email to a list of prospects that are assigned to several different sales reps, and having each prospect receive the email from their assigned rep, is to select Assigned User for the Sender of the email. This option will use the email address and name of the user who is assigned to the prospect in Salesforce or Marketing Cloud Account Engagement as the sender of the email. This way, the email will appear more personalized and relevant to the prospect, and will increase the chances of engagement and response. Selecting General User, Account Owner, or Specified User will not achieve the same result, as they will use a generic, fixed, or predefined sender for the email, regardless of the prospect’s assigned user. References: [Salesforce documentation]