Currently, in order to allow an email to be sent via the prospecting agent. The user needs to go to Settings → General → Email → Enable Agent Access.
The issue is, even to have that setting, users must have the following permission
"Prospecting agent assignment
Assign contacts and companies to the prospecting agent for monitoring and outreach"
There should be a way where we can allow the email to send emails via the prospecting agent while preventing non-technical users from assigning contact to it.