1. Log into http://webmail.tncsa.com 2. Click Settings 3. Click Additional Settings 4. Retype your login credentials (email address and password) 5. Under Junkmail Quarantine, click Configure Junkmail Settings 6. Add @domain.com where you want any email from that domain to be able to pass through or just the particular email address(es). Be sure to whitelist those you want to come through and choose blacklist for those you don't. Leave the level as is and click Add.

Here are more details if you want to go deeper.