Outlook has a feature using which voting options can be enabled for a email message. You can enable voting feature using the settings available as parts Message Options menu. Listed below are the steps to turn on voting option for email messages in Outlook 2013, Outlook 2010 and Outlook 2007. Click on the New E-mail icon and navigate to Options menu.
Click on the drop down arrow available below the Use Voting Buttons menu. This would display the different Voting options that are available such as Approve;Reject, Yes;No, Yes;No;Maybe. If you want to customize the voting text then click on Custom from the menu list and provide the customized text (example:- High;Low) in Use voting buttons drop down under the section Voting and Tracking options in Properties window.
The Voting buttons enabled message would look as shown below
Also See: How to recall email message in Outlook