Open the System User and assign the license for Exchange Online
Open the system user record and click on Open Mailbox to open user’s mail box.
Set Server Profile as Microsoft Exchange Profile and Server Side Synchronization or Email Router option for Incoming and Outgoing email.
Approve Email command.
Test and Enable Mailbox command
On successful configuration all the warnings will be gone
Go to Settings à Email Configuration
Click on Email Server Profiles
Select and open the already existing Email Server Profile i.e. Microsoft Exchange Profile.
Open Mailboxes and select Test and Enable Mailboxes to test the mail boxes of the user.
Create an email activity and send.
Open Outlook to verify.
Hope it helps..