If your email was migrated from another service (e.g. Gmail, Exchange, Office 365), all of your email is in your email system. Since it is a different system than before, your mail client (e.g. Outlook, Apple Mail, Thunderbird), considers it a new message, especially since each message will have a different ID.

In general, we recommend creating a new email account in your mail client. If using IMAP, the initial synchronization will be faster for a new email account than from an account your mail client has been using and has to reexamine all folders and messages. If using POP, you may want to consider archiving your previous mail in your mail client, and setting up a new account to download mail again.