The difference between how your email storage looks in the mail client and how it is actually stored in your computer is very big. If you look into Gmail, for instance, you will see the emails sorted out into folders like Inbox, Sent, Drafts, etc. On the hard disk all this will be stored as a one file, in Mbox format.
Such approach provides faster performance, as storing each message is a separate file makes it longer to call all the files and update the information when new emails arrive and old ones are deleted.
Mbox message storage is like a big text file (7-bit ASCII), where all the information, including attachments, is encoded in a specific way. Each message starts with 'From' line that contains the address of sender. After 'From' an empty line usually goes. This is how one message is separated from another in MBox storage.
The file is updated each time you get new emails or send them. To know what folder the message belongs to, there is also service information is recorded to MBox storage.
If you found MBox file in your hard disk and would like to read its content, you will need to import the file to mail client that supports this format or use MBox converter. Otherwise the messages stored there will not be readable.
Remember to take a look at Kite MBox Converter when you want to convert your MBox emails to PDF, DOC, TXT, TIFF, HTML files.