To whitelist an email address, add it to your safe senders list in your email client's settings, create a filter to "never send to spam," or simply add the sender to your contacts. The specific steps vary by provider, but most involve navigating to the settings or filters section and entering the email address or domain.

For Gmail

Click the gear icon and select See all settings.
Go to the Filters and Blocked Addresses tab.
Click Create a new filter.
Enter the email address in the From field and click Create filter.
Check the box for Never send it to Spam and click Create filter again. 

For Outlook

Go to Settings (the gear icon) and select Mail.
Click on Junk email.
Under Senders, select the Safe senders and domains tab.
Click + Add safe sender and enter the email address or domain.
Click OK and then Save.
Alternatively, open an email from the sender, click the three dots, and select "Block > Never block sender". 

For other clients

Add to contacts:

 Many email clients will automatically send emails from a new contact to your inbox. Manually adding the address to your contact list is an easy way to whitelist it.

Move from spam:

If an email lands in your spam folder, select it and click "Report as not spam" or move it to your inbox. This trains the system to recognize the sender as legitimate.