An email becomes a record when it documents a decision, approval, advice, commitment, or action. Capture important emails into the approved system; a personal mailbox is not long-term storage.
An email is a record when it carries evidence you would need later: a decision, approval, advice, commitment, or action. Those emails belong in the approved recordkeeping system; a personal mailbox is not long-term storage and is easily lost when someone leaves.
No. An email is a record when it documents a decision, approval, advice, commitment, or action. Routine or trivial messages generally are not, though your organisation's rules apply.
IAR turns terms like this into working practice: assets classified against your disposal authority, stewarded by position, and ready to evidence.