MailItem.FlagRequest Property (Outlook)
Returns or sets a String that indicates the requested action for a mail item. Read/write.
By default, a mail item is not marked with any flag and the default value for this property is the empty string. You can set the value of FlagRequest either through the user interface or programmatically. When you mark a mail item with a flag through the user interface, the default value of FlagRequest is "Follow up".