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Mailman 3 has a whole new interface and more importantly, there is no longer a shared password to mange teach each list. If you are a list admin you need to to setup an account on the mailman 3 system to access your list.

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Mailman 3 lists will work as normal, But you will not be able to do do any administrative functions  on your list until you create an account in the Mailman 3 system. If you admin more than one list, your account will get you access to all the lists you admin.

To create an account go to http://mailman.cchmc.org/ and click the sign up button.

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