All email addresses entered in to Sugar are stored in the 'email_addresses' table. The email addresses are then related to the different record types, Accounts, Contacts, Users, etc. via the 'email_addr_bean_rel' table.
Here's a sample query that would return the email address of the System Administrator User with ID 1:
SELECT ea.email_address FROM email_addresses ea JOIN email_addr_bean_rel eabr ON ea.id = eabr.email_address_id JOIN users u ON eabr.bean_id = u.id WHERE u.id = '1';
