Trying to understand back up and import of emails

[EricJH]

I am trying to understand how the backup and restore of email works with the Mails folder and MailDB in relation to what is downloaded from the server. I am partially leaning on the information in this post . I am wondering what is being downloaded.

In the situation of a clean installation I see two scenarios:

1. Only restoring the Mails by copying the Mails folder to the new Default folder and then setting up the email accounts
2. Restoring both Mails and MailDB by copying the Mails and MailDB folders to the new Default folder and then setting up the email accounts

I noticed that the number of the folder that holds the mails of a certain account is always the same for a given email account regardless of installation. I guess the number is a simple hash code.

Scenario 1 - only restoring Mails
What happens after the email accounts have been set up?

1. The email client will fetch the headers and will not download the bodies of the emails that are already there and only will download the bodies of new emails?
2. It downloads both the headers and the bodies of all emails?

Scenario 2 - restoring Mails and MailDB
What happens after the email accounts have been set up?

There is one question that precedes. With a local index will the email client compare local and remote indexes to decide what headers and bodies are downloaded? Is that a correct understanding?

1. Will the email client compare local and remote indexes and will only download headers and bodies of new emails?
2. Will the email client compart local and remote indexed and will download bodies of all emails?