Hello,
Just sharing the solution I received from support:
This icon denotes that you have a quoted text. When you have replied to an email, the previous conversation will be collapsed automatically and it will be expanded by clicking on the icon.
To disable this, please execute the below query in the database,
update globalconfig set paramvalue='true' where parameter='Show_Quoted_Content'
* Application restart is necessary after this change.
Demetrius