I just created a project for each mailbox and set the passwords/servers inside each project. Each box is scheduled for the same time period, but since they are different boxes they stagger somewhat, reducing load. ( we have several thousand tickets a day come in that way )
I have 4 'create' mailboxes, 2 'close' and 2 'update'. The reason for so many is that our external vendors weren't able to accommodate the collapse into a single box so i was stuck with the extras. Internally we did it several years ago.
Ours are fed mostly by internal web pages, and we insert a special code into the body of each mail that i can key off of to determine the appropriate category. We did that with separate mailboxes in v6 early on, until it got way out of hand so i instituted the 'rule' policy instead and reduced the number of mailboxes.