For listserve aliases with custom domains, you need to update your MX records using your DNS tools:
list.yourdomain.org would point to org-lists.salsalabs.com (or org2-lists.salsalabs.com or afl-lists.salsalabs.com, depending on your node)
Once that is set up, you can go here to the following page to test to make sure it is set up correctly: http://www.mxtoolbox.com/
Then, for each of the listserve groups, copy the full default listserve address referenced at the bottom of the listserve tab, then click on the "Edit Alias" link. You'll redirect to a new page where you can use the Add Alias button to add another row on the Alias list.
Paste the default listserve email address into the Real Address field, and type the desired Alias address into the Alias. (As an example, XXXX_Group_Listerve@org-lists.salsalabs.com would be the default listserve email address, and GroupName@list.yourdomain.org would be the alias).
When you've made the updates on your end, please email us at email@example.com and we'll make sure our systems are updated to reflect your changes as well.