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