Yes there must be an Account in your AD with a mailbox.
alternatively you can have just 1 mailbox for inbound emails and outbound or have a seperate mailbox for outbound.
But each email address you want it to use must exist on your server.
Hope this helps.
I assumed that would be the case but it's worth checking
Do you know if it clears the mailbox down when it polls the messages so that the mailbox doesn't get to quota or does this have to be done manually?
No it doesn't clear down the mailbox.
However depending on your version of exchange you can do certain things to assist with this.
For example on exchange 2003 you can create a mail store that is set to delete items older than 2 weeks.
It has to be a seperate mail store as it takes effect on all mailbox's on the mail store.