As a site administrator, when a list owner wants to use their own subdomain
(list.theirdomain.com) with their list, we instruct them specifically *not*
to have an MX record associated with the subdomain, but to *only* have an A
record created and pointed to our server's IP. In our experience, if there
is an MX record, mail sent to the list address bounces. Of course, it's
possible we are not properly configuring something on our end, but without
MX records,  the subdomains work perfectly with our customers' lists.
Therefore, I have concerns that creating MX records to satisfy Earthlink is
going to create more problems than solutions.

Michelle