Attaching Email reply to a contact in Civi
See https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/#autofiling-email-activities-via-emailprocessor where you can set up either a mailbox you can bcc that will add email replies to contacts, or you can set it up to pull from a specific folder in your own mailbox after-the-fact to do the same thing.
In terms of the earlier part of your question, when you say &quot;contacts within orgs&quot;, Civi won't send to contacts who are employees of orgs, but if you set up Groups, then you can mass email to the group. See https://docs.civicrm.org/user/en/latest/email/mass-mailings-using-civimail/