MSO file format is used by email service providers. This file is used to define the structure of an email when any attachment is added to the text based email.

The information related to the position of the attachment with respect to the text of the email is mentioned. The HTML and plain text based emails do not support this attachment configuration and hence MSO file is not used in these conditions. Only rich text based emails are able to support this formatting option of the attachment with text. This file is generally used by Microsoft Outlook to define the rich text based emails.