I think you are referring to the Archive To Go Viewer. When the Viewer displays an HTML-format message that includes an active link to an email address (ie. mailto:), you can click the link to have your default email client open a new composing message with that same address as the recipient. The Viewer does not support active mailto: items in the plain-text format.
Archive To Go 1.2 introduced Forward and Reply for archived messages. Which version are you using?
Could you provide more details on how you wish to use mailto: with Archive To Go?
