The junk mail filter built into mainly email client like Outlook, adding known senders (senders email by adding known senders to its list of Safe Senders for email receiving accuracy. This makes sure mail from known senders always goes directly to your Outlook Inbox, no matter what the junk mail algorithm might think.
You can also whitelist complete domains using Safe Senders adding Domain to Safe Senders in Outlook
For more convenience, please check the step by step picture help;
To add an address or domain to Safe Senders in Outlook:
Select Actions | Junk E-mail | Junk E-mail Options... from the menu in Outlook.
Go to the Safe Senders tab.Click Add....
Type or copy paste the following domain name to whitelist.
Please keep in mind, sub-domains are not automatically whitelisted when you add a domain, include the '@' sign: "@smdrhk.biz", for example. In this case, mail from "email@example.com" will not be whitelisted.
Click OK again.
If you already have a message from the sender you want to add to the Safe Senders list in your Outlook Inbox (or the Junk E-mail folder, of course), the procedure is even easier:
Open a message from the sender (or a sender at the domain) you want to add to Safe Senders.
Select Actions | Junk E-mail and then Add Sender to Safe Senders List or Add Sender's Domain (@smdrhk.biz) to Safe Senders List.