Signing into the same email account on a replacement Mac does not establish that every mailbox from the old computer will reappear. Apple Mail distinguishes server mailboxes from folders stored only on the Mac. Identify that boundary before the old machine leaves service.

Open Mail’s sidebar and expand On My Mac and each account separately. Write down the names of locally stored project and archive mailboxes, including nested folders. A mailbox’s familiar name does not reveal where its messages are stored.

Compare location with actual availability

Apple states that On My Mac mailboxes are local to the computer, while mailboxes on the account’s server are accessible from other devices using that account. Select a representative message from each important local folder and record enough non-sensitive detail to recognise it later.

Check the same account through its normal webmail or another configured device. The purpose is to establish which project records are actually available there. Avoid assuming a folder is complete simply because one recent message appears.

In Mail, Settings, Accounts, inspect Mailbox Behaviours for the relevant account. Where supported, Drafts and Sent can point to either server or local mailboxes. Settings vary by account type; record the current arrangement before changing anything.

Build a migration inventory

List each required mailbox, its location, approximate date coverage and any important attachments. Flag items visible only on the old Mac for a deliberate export or other supported transfer procedure. Keep personal and organisational material separate according to the intended handover.

Do not delete a mailbox as a test. Apple warns that deleting an ordinary mailbox deletes its contents and cannot be undone through that operation. A location audit requires observation, not cleanup.

After the replacement is ready, use the inventory to check older as well as recent messages and attachments. Retain the old Mac and its backup until those checks pass. Successful account sign-in and successful preservation of local archives are separate results.

References: Apple: mailbox locations, Apple: mailbox behaviours.