Most of the major email service providers have a built-in spam filtering feature. Because the spam filtering feature is automatic, it is possible for important messages to accidentally be routed to your spam folder. When this happens, you must whitelist the domain name that is being marked as spam. If you don't want to whitelist an entire domain name, you can whitelist a specific email address.
GMail
Log into your GMail account. Open your "Spam" folder.
Locate the message for the domain you want to whitelist. Place a check in the box located next to the message.
Click "More Actions." Select "Filter Messages Like These."
Enter into the "From" field the domain name you wish to whitelist. You can enter one specific email address or the entire domain name. For instance, user@blockeddomain.com or @blockeddomain.com.
Click "Next Step." Place a check inside the box labeled "Never Send it to Spam." Click "Create Filter" to save your changes.
Hotmail
Log into your Hotmail account. Click your "Inbox" folder.
Select the "Options" link on the upper, right-hand side of the page. Click "More Options."
Look underneath the "Preventing Junkmail" section. Select "Safe and Blocked Senders."
Click "Safe Senders." Enter the domain name you want to whitelist. Click "Add to List." The domain will automatically be added to your whitelist.
Yahoo!
Log into your Yahoo! email account. Click the "Options" link in the upper, right-hand corner. Select "More Options."
Click the "Filters" option from the left side column. Select the "Add Filter" option from the Yahoo! top navigation bar.
Add a name in the "Filter Name" field to help you distinguish this filter from all other filters. In the "Sender" field, select "Contains" from the drop-down menu.
Enter the domain name or email address you wish to whitelist. In the "Move to Folder" field, select "Inbox."
Click "Save Changes" from the Yahoo! top navigation bar.