Hi Quad,
Are you saying you accidentally deleted the default list of domains on the Spam -> Sender Authentication page?
Just so you know, Symantec will actually be changing the list that currently exists in the product as not all of the domains in the list have an SPF record with the "-all" directive which is required for a message to get a sender authentication failure verdict.
The list of domains that will be included with the next version of the product will be:
citimortgage.com
discovercard.com
email.chase.com
digg.com
about.com
redhat.com
blogger.com
spamhaus.org
friendster.com
uol.com.br
worldonline.de
Also here is a handy website to have, if you see emails being spoofed from a domain, check to see if they end with "-all", if they do add them to your list and that should end your spoofing issue.
http://spf-all.com/
I woud say it's probably worth testing all the domains in the list occasionally to make sure they still have the "-all" records and haven't changed them, if they have changed them it's probably worth removing them from your list.
Hope that helps,
Kevin