Forms will autofill with constituent data after users click links in email messages, if Use email links to autofill forms is marked under the Administration settings in NetCommunity. This setting only works for email messages that sent to query-based email lists that use constituents as the data source. You create the queries in The Raiser's Edge and then create email lists based on the queries in Lists. For information about email lists, see the Email Guide (PDF).

Warning: If a recipient forwards a query-based email and someone else follows a link from the email message to a web form, the original recipient's biographical details autofill on the form.
When enabled this Email Settings feature within NetCommunity autofills a user's information into donation forms accessed by a link in an email sent from NetCommunity.  However, if the email is forwarded to another user, the initial receipent's information auto-populates when clicking the link, since that link is specific to the intended recipient. The setting autofills forms with biographical details such as name, title, primary address, phone number, and email address. When you include links to forms in query-based emails, the program pulls data from constituent records even when users are not logged in to your website.
The setting applies to forms that you create with Donation Form, Email Forwarding Form, Event Registration Form, Event Registration Form (Classic), Membership Form, User Email Preferences Form, and User Login parts. It does not apply to forms that require users to log in because email links do not bypass login requirements.

The only way to prevent this from happening for forwarded emails is to turn of the auto fill feature in Administration > Sites & Settings > General > Email Settings.

Note:  Disabling this setting will affect new emails. It is not a retroactive change.