I am not sure exactly. I would imagine that by the 1970s or 1980s it was. I used to work for a different mail forwarding company than either Outbox or Earthclass Mail and worked on an internal version of the mail imaging/OCR/barcoding/etc machine.
The papers that I read detailed the efforts of folks to make OCR software primarily for the USPS for mail sorting machines. The earliest papers I saw were early 80s. You can read more about the history of mail sorting here: http://en.wikipedia.org/wiki/Mail_sorter
I would tend to assume that the advances in mail sorting roughly coincided with bulk mailing but I can't point the causation arrow. It could be that bulk mailing really took off and the USPS got swamped and then people started making mail sorting machines. Or it could be that people invented the machines, the USPS then rolled them out and then you saw presort discounts and then bulk mailing took off. I'm not sure which.
