Currently O365 provides the ability within Transport Rules to prepend a message to your subject or body. We currently have a rule for all incoming emails to be tagged with [External] in the subject/body of the message.
If we can prepend something into the subject/body of a message, why can't Exchange Online natively remove that tag upon it's outgoing process? From my research there are not many products that can complete this task unless using centralized hybrid mail flow back through On Prem.
Could the product team looking into adding this feature for Transport Rules?
This is huge. Many are moving to this to help the users clarify if something is internal or external. I cannot believe this has not been implemented yet. Please do this!