Mail merge and folding the document

We are able to mail merge our newsletter and it works great. However, the merge creates a huge document, with each addressee having its own set of pages. When we send it to the printer (Konica Minolta Bizhub C550), it sees a single document of (6 pages x 300 addressees = 1800 pages) and refuses to fold each individual newsletter.

How can we tell the printer to only fold six pages at a time?

If we print each copy individually, it works fine (i.e., print pages 1-6, then print 7-12, etc).

This also occurs if we try to staple a similar document.

We're using InDesign CS6 and MS Windows 7.

Thanks!