If your old provider supports IMAP, you can copy the whole mailbox into InfoPeak.
- In Mail, open Mail settings and choose Migration.
- Enter the IMAP server of your old provider, your old address and its password. Many providers require an app password for this.
- Start the move. It runs in the background, and you can check back later.
If the move cannot start, check the login details. Moving from Gmail, Outlook, iCloud or Proton? Use the guides for those instead, they are easier.