Hello,
To allow Users to send emails from your ERPNext account, you should ensure that Roles assigned to those Users also cover Email permission on that Doctype.
For now, ERPNext doesn’t have complete email client feature. Currently, you can setup a global/default Outgoing Email Gateway for your ERPNext account. Though the recipient will be able to see User ID who actually sent that email, but email will be delivered from the default outgoing gateway only.
https://frappe.github.io/erpnext/user/videos/learn/email-account.html