A mailbox export should be tested before it becomes the reason to retire a computer or close access to an account. A folder ending in .mbox proves that an export was created; it does not establish that the messages and attachments you need can be read from it.

In Mail on Mac, select the intended mailbox or mailboxes and choose Mailbox, Export Mailbox. Save into a new, dated folder whose name identifies the account and scope without exposing private message content.

Keep each export attempt identifiable

Apple exports mailboxes as .mbox packages. If a previous export exists, Mail creates another package with a modified name rather than overwriting it. Record which folder belongs to the current attempt so a later test does not accidentally examine an older export.

Keep the source account and mailboxes intact. Choose representative records before testing: an older message, a recent message, a long conversation and examples with the attachments that matter to the project.

Use File, Import Mailboxes in a suitable controlled Mail environment. For a mailbox exported by Mail, select Apple Mail as the source and follow the displayed instructions. Apple places the imported messages in a new Import mailbox.

Read from the imported copy

Open the imported mailbox and locate your representative records. Confirm their dates, participants and content. Open saved attachments in their appropriate applications where safe and expected; seeing an attachment icon alone is not enough.

Compare the date range and folder coverage with your export inventory. A count difference deserves investigation, but matching counts do not prove every message is the correct one.

Keep the verification separate from live mailbox organisation. Do not drag imported records into active server folders simply to see whether they work, because that can create a second set of messages.

Store the accepted export with its scope and test notes, and protect it as private correspondence. Retain the original mailbox until the wider migration or retention decision is complete. An export can preserve selected records without preserving every account setting or future incoming message.

References: Apple: importing and exporting mailboxes.