Create a Safe Sender Filter (Most Reliable)

• Open Gmail on a computer and click the Gear Icon (Settings) in the top-right corner.
• Click See all settings.
• Navigate to the Filters and Blocked Addresses tab at the top.
• Click Create a new filter.
• In the From field, type the sender's email address or a domain name (e.g., @example.com).
• Click Create filter.
• Check the box for Never send it to spam.
• Click Create filter again to finish

Add to Google Contacts

• Go to Google Contacts.
• Click Create contact in the top-left corner.
• Enter the sender's name and email address.
• Click Save.

Note: In our testing we found that Add to Google Contacts was enough.

Create an Inbox Filter (Most Reliable)

• Click the Settings (gear) icon in the upper-right corner and select More Settings.
• Click Filters on the left menu.
• Click Add new filters and give your filter a name (e.g., "Safe Sender").
• Under the "From" section, enter the sender's email address.
• Under "Choose a folder to move to", select Inbox, then click Save

Add to Contacts (Fastest)

• Click the Contacts icon on the right side of your Yahoo Mail inbox.
• Click Add a new contact.
• Enter the sender's name and email address.
• Click Save.

Note: In our testing we found that Add to Contacts was not always enough. Email would still be moved to spam folder.
we had to locate the email, click Not Spam. Sometimes 2-3 times before Yahoo finally accepted and put it in the Inbox.

On iCloud Mail (Web)

• Log in to your iCloud Mail account.
• Click the Settings icon in the corner and select Settings.
• Go to the Rules tab and click Add a Rule.
• Set the condition to is from and enter the email address.
• Set the action to Move to Inbox and click Done

Note: Only info found how to add to safe sender without already having an email from OCS.

On iPhone or iPad

• Open the Mail app and locate an email from the sender.
• Tap the sender's name or email address at the top of the message.
• Select Create New Contact or Add to Contacts.
• Tap Done to save.
• Alternative: If their email is already in your Junk folder, you can simply swipe left on the email,
  tap More, and choose Mark as Not Junk

On Mac (macOS Mail App)

• Open the Mail app
• Open an email from the sender you want to trust.
• Hover your mouse over the sender's name or email address, click the dropdown arrow that appears,
  and select Add to Contacts.
• Alternative: If you need more advanced control, you can create a whitelist rule.
  Open Mail > Settings (or Preferences) > Rules, add a rule, and set it to move messages From your specific sender directly to your Inbox