Log in to the Webmail Portal.
Click the Configuration icon toward the top-right of the page and select Edit Settings.
1&1 WebMail start page
In the Configuration menu, select Anti-SPAM from the Mail section and then click Show beneath the Whitelist option.
Click the Add button.
Type in the e-mail addresses or the address range in the provided textbox. The address range will allow you to filter any e-mail address containing a specific domain name as seen in the image below. When finished, click on the Add button to finalize the procedure.
The newly whitelisted address will automatically be saved.