The email on away plugin allows to forward messages to user's email address when the user is away (not offline).
Copy the emailonaway.jar into the plugins directory of your Openfire installation or install it via Admin Console. The plugin will then be automatically deployed.
Configuration can be changed and tested in Admin Console > Server Manager > Email settings. To not show email in the automated message create a system property "plugin.emailonaway.showemail" with the value "false".