Ask HN: Simple Online Helpdesk Ticketing System?

We recently decided with some of our clients we need to use a ticketing system, as the pure volume of emails they send us with support requests and feature requests, it is is too easy to lose an email. So we're looking for the most simple system that satisfies the following requirements.

* Can parse emails+attachments and create ticket

* Allow web submitted tickets with different priorities

* Multiple users per ticket(ie two of the clients users can be on the ticket)

* Let user see their own tickets and tickets from their company(based on their email domain).

* Export tickets for a given company by one of their users.

We'd like internal notes on tickets, but that isn't a must. We looked at osTicket which has a lot of these features, but it doesn't have the view all tickets for a given client(as a client user) or export ticket.

Kayako we looked at, but it may be a bit over kill.

Open source is great, free is even better.

Any suggestions?