Configuring Inline Protection Settings for Exchange Online

Go to Administration > Global Settings > Inline Protection Settings for Exchange Online.
To turn off Inline Protection for Exchange Online without deprovisioning for Exchange Online (Inline Mode), disable Inbound Protection and Outbound Protection under Inline Protection.
To use the message header address for matching the approved or blocked sender lists in the ATP policy, select Message header address under Sender Address Match.
The envelope address is always used for matching the lists.

If you have also provisioned for Exchange Online, to avoid scanning an inbound message twice with the same security filters, enable Trust Scan.

Trust Scan applies only to a security filter that is enabled for both Exchange Online and Exchange Online (Inline Mode) - Inbound Protection.

Table 1. Email Scanning by the Same Security Filter
Protection Mode	Scanning Time	If Trust Scan Disabled	If Trust Scan Enabled
Exchange Online (Inline Mode) - Inbound Protection	When email in transmission	Scan	Scan
Exchange Online	When email arriving at the mailbox	Scan	Not scan

Note:

Trust Scan applies to real-time scanning only.