We have the same issue. Due to evolving internet standards regarding spam, doman abc.com can not send an email addressed From: user@xyz.com. It has to be sent From: user@abc.com with Reply-To: user@xyz.com.
When osTicket receives an email, perhaps it should check for the existence of a Reply-To address and use this as the User, or have an option in the setup to choose this action.
I'll try a filter.