Here is a step-by-step guide on how to do this based on your email provider
Log into your Gmail account, locate the Settings Gear icon, and click it. You should now select Settings from the resulting drop-down menu. Navigate to the Filters and blocked addresses tab. Click Create a new filter and paste firstname.lastname@example.org into Never send it to spam. Click Save, and you're done. You can also choose to Star the emails, apply a given label, or mark them as important.
Log into your Gmail application and navigate to Spam or Junk Folder. Click on a message from email@example.com, and select the option Report not spam. You will now receive messages from us as normal.
Log into your Outlook account, locate Settings, and click View all Outlook settings. Go to Junk email and choose Safe senders and domains or Safe mailing lists. Click Add and paste firstname.lastname@example.org. And you're done. You can also simply add email@example.com to your Outlook Contacts from an email we sent to mark it as safe.
Log into your Outlook Mobile App, select a message firstname.lastname@example.org. In the message click on the three dots in the top right corner and click Move to focused inbox. When the pop-up screen comes up, click Move this and all future messages.
Log into Yahoo! Mail, and locate a message from email@example.com. Highlight that message in the Bulk folder and mark it as Not Spam. And you're done, the Yahoo Mail spam filters will automatically allow future messages into your inbox.
If you haven't received an email from firstname.lastname@example.org before. Navigate to the Settings icon, then click More Settings from the Yahoo Mail menu. Select Filters and hit Add to enter email@example.com.
Log into Yahoo Mail Mobile App, locate the sidebar and click the Spam folder. Find an email from firstname.lastname@example.org and Click Move and then click Inbox for the Yahoo Mail spam filters to automatically allow future messages into your inbox.
Desktop and Mobile App:
Log into your AOL account and open Contacts from the left navigation pane. Click the New Contact icon and enter email@example.com. Click the Add Contact button located at the bottom.
Desktop: Log into iCloud Mail and select Contacts. Select the + symbol at the lower left and click New Contact. Enter firstname.lastname@example.org, and click Done. All future correspondence should now end up in your inbox.
Mobile App: Log into iCloud Mail and create a Contacts. Tap the + symbol in the top right and add email@example.com. Select Done in the upper right corner and that's it.