Hello,
The e-mail you set up during epesi is the e-mail to be used to send out new user information and other system messages.
To edit these settings, you need to log in as Administrator, head to Menu->Administrator->Mail server settings.
If you want to use the MailClient that is included in epesi, head over to Control Panel (if you're using fresh 1.1.7, you may need to select My Settings->Change Password, then "Back" to reach control panel) and check out E-mail Accounts. After setting up the account, you'll have access to is via Menu->E-mail.
Make sure module CRM/Roundcube is installed to have this option.
Hope this helps,
Arek