From a web browser, navigate to http://mail.cass.net

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At the login screen, login with your full email address and password.
Once logged in, on the left hand side, click on ‘Manage Mailboxes’.

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On the far right side, click the blue ‘New Mailbox’ button.

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In the Email Address field, type in what you would like for your new address.
Then type in a password, and confirm the password in the box below.
Once finished, click the ‘Create Mailbox’ button.

See also: Tips for creating a strong password