A whitelist is a list of approved or safe email addresses to help a spam filter determine whether it should allow delivery. If you are on your recipient's whitelist, you can guarantee your emails will be received. If you want to make sure you receive messages from a certain person or company, you simply need to manually add them as a contact.
In Gmail, whitelisting an email actually involves creating a filter.
The "From" field is where you'll specify what's getting whitelisted. In Gmail, you can specify either a specific email address (e.g. [email protected]), or you can whitelist a whole domain by omitting everything before the at symbol (e.g. @domain.com). The former will whitelist one specific email address while the latter whitelists anyone who is sending emails from that domain.
From there, you don't need to input any additional information. Simply click the Create filter button at the bottom of the window (next to the blue Search button).
Clicking the button brings up a new window. Here's where you'll actually specify that what you're doing is whitelisting. In Gmail, you do this by checking the Never send it to spam box.
Doing so will ensure that emails that meet the criteria you specified never end up in Spam or Trash (unless you do so manually).
Once this information is specified, you click the "Create filter" button again, and this creates the whitelist within Gmail.
When you select it, a drop-down menu will appear. At the bottom of the drop-down menu, you'll choose View all Outlook settings.
This will save your whitelist preferences and ensure that emails that fit the criteria don't make it to your junk email folder in Outlook.