To manually migrate from Zimbra to mbox, you can follow these general steps:

Export your Zimbra mailbox: Log in to your Zimbra account and export your mailbox data in the .tgz format. You can do this by selecting the desired mailbox or folder, right-clicking on it, and selecting the "Export" option.

Extract the .tgz file: Extract the .tgz file using a program like WinRAR or 7-Zip. This will create a folder containing the mailbox data in various formats, including mbox.

Convert Zimbra mailbox data to mbox format: You will need to convert the Zimbra mailbox data to mbox format, which can be done using a third-party tool like Aid4Mail or mboxConv. Follow the instructions provided by the software to complete the conversion.

Import the mbox file: Once the Zimbra mailbox data is converted to mbox format, you can import it into your desired email client. Most email clients have the ability to import mbox files directly, or you can use a third-party tool like ImportExportTools for Thunderbird.

Verify data integrity: After importing the mbox file, it's important to verify that the data has been migrated correctly. This may involve spot-checking data and comparing it to the original source.

Regards,
Peter