The header is written like that to make it easier to track spam being sent from your server. You can stop email being sent ‘on behalf of’ notice in Microsoft outlook from your WHM interface. To do this, please follow the below given steps:

1) Login to your WHM interface.

2) Navigate to Service Configuration >> Exim Configuration Manager.

3) Click on the option ‘Mail’ under ‘Basic Editor’ category.

4) Scroll down the page and find out the option ‘Set SMTP Sender: headers’. This option will create “On behalf of” notices in Microsoft® Outlook, but it may also help track abuse of the mail system since recipients will see the SMTP login used to send each message.

5) Change the option ‘Set SMTP Sender: headers’ from ‘On’ to ‘Off’ to stop this feature.

6) Finally, click on the ‘Save’ button to save the changes.