I have the same setup with a support email address (Email A) and an alerts email address (Email B).
I never want users to see the alerts email (Email B), so setting the Department Outgoing Email to Email B is not an option.
Setting the Default System Email to Email B doesn't fix everything either, because then the Department Outgoing Email must be set to Email A and so new ticket alerts still come from Email A.
Since new ticket alert emails always come from Email A, auto-responders from agents who go on leave create an avalanche of new tickets. To stop this, I've had to create a filter to stop new tickets from my own domain, which is an ugly workaround that limits what agents can do with osTicket. Is there a better solution for this problem?
I expect the same email from-address behavior as @konst643 describes above with his screenshot, but that clearly is not how it works. Could we get some clarity as to which emails (the ones in the screenshot) use which address? (I appreciate they have a fall-through of multiple addresses, so a list of which function each uses would be helpful as a simpler alternative.)