实测发现，osx mail export 最大只能到 2.42G, 大概是 4000 封左右正常邮件的量(实测4348)，超过该数量也只能导出 2.42G, 然后用该导出文件导入的时候报：

Some messages could not be imported. The partially imported mailboxs are located in the mailbox named 'import' in the mailboxs list.

OSX: macOS High Sierra(Version: 10.13.6)
Mail: Version 11.5