If You Use Gmail...

​To make sure Gmail never filters YourDomain email as spam, all you have to do is mark the conversation as important.

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If You Use Outlook...

To make sure Outlook never filters YourDomain email as spam, take the following steps:

Step 1: 
Select the message of the sender you wish to Whitelist  

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Step 2: 
Then click Home > Junk > Never Block Sender OR you can right click the selected message and choose Junk > Never Block Sender from the drop down menu  
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​Step 3:  
You will be prompted with a dialog box notifying you that the selected address has been added to your safe senders list 

​If You Use Mac Mail...

To make sure Mac Mail never filters YourDomain email as spam, take the following steps:     
            
Step 1:
Select Mail > Preferences from the menu in Max OS X Mail

Step 2: 
Select the Rules category and click Add Rule

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Step 3:  
Type a description and ensure the criteria reads correctly 

Step 4:  
Enter the domain name you wish to whitelist, and enter the actions you want to be performed 

Step 5:  
Click OK