CoreBOSBB

I wish setting field above with specific value, when ticket is created thru mail scanner

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At first, one possible solution is created a new and specific user, and setting a workflow to rewrite this fields

Is there another way?

That is the way to do it. We added a field on the ticket called "From Mailscanner" so you can create a workflow and set the condition on this field to detect tickets coming from the converter and then use the tasks to do whatever actions you need to take.