In order to ensure that specific emails are marked as spam or not, Exchange Online or Exchange Online Protection (EOP) support the use of transport rules, to make whitelists or blacklists, and control how messages are processed, whether if you need to bypass spam filtering in order to prevent good email messages from getting marked as junk mail, a false positive, or ensure that spam messages are placed in a quarantine folder.
An alternative method is to use the Spam filter settings in EOP, but these must be assigned to specific users, domains or groups where transport rules applies to all messages. Using a transport rule will then often be a faster and easier method to manage allow and block listing of messages in EOP,
This guide will focus on making a white-list for specific external sender addresses that should be delivered in all cases. Depending on the requirements, the conditions of the whitelist can easy be altered to allow all senders from an entire mail domain.
To access this feature, in the Exchange admin center, click mail flow, and then click on rules
Open the drop-down list at the plus sign, and select Bypass spam filtering…
Enter the rule Name (in this example [Custom] Whitelist)
Open the drop-down list under the Apply this rule if… option, select The sender… and then select is this person
If the scope is an entire mail domain, instead select The sender… and then select domain is.
In the Select Members page, enter an external sender address in the lowest text field, and click on check names to add the sender adress.
Repeat for any additional sender addresses, and then click OK
Click on add condition
Open the drop-down list under the new option, select The sender… and then select is external/internal
In the Select sender location box, select Outside the organization, and click OK
Scroll down and enable the Stop processing more rules option
Scroll further down and set the Match sender address in message to Header or envelope
Review our new white-list rule.
Over time the sender addresses in the rule may need to be changed according to current requirements.
Just select the rule and click Edit (the “pencil”) and click on the existing addresses, to add or remove sender addresses.