The e-mail pass through feature allows you to use multiple incoming and outgoing e-mail addresses to provide better user experience and branding. When the pass through is enabled the helpdesk system will send replies from the e-mail address used to receive the ticket in the first place.
The feature is available with the Premium billing plan and allows you to easily consolidate multiple helpdesk accounts into one. Here are some cases where this is useful:
A product could have different brandings and could be offered on different sites with different domain names. In this case the replies will be sent from the proper domain that will match the domain where the ticket was originally received.
A company could have different products offered on different sites with different domains. Sending replies from the proper domain would offer better customer experience and the company doesn't need to have separate helpdesk for each of their products or brands.
A support team could provide support services on behalf of several companies. Normally a helpdesk would have to be created for each company but with the e-mail pass through the team can use a single helpdesk.