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 Email 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.
For additional information, you may want to reference: