When you first activate Email Archive, the tool will prompt you to choose a site email address. If you've already activated Email Archive and chosen a site email address, you can edit it within the Options area.
See How do I add tools to my site? for initial steps to activating the Email Archive tool using the in Site Info tool.
While activating Email Archive in Site Info, the Customize tool instances area will prompt you to choose an address:
Click Finish to confirm the activation of Email Archive and the selection of the site email address.
The site email address, when already chosen, can be edited in the Options area of the Email Archive tool. See How do I modify the Email Archive options? for complete steps.