In fact, it was ibm that was the mail carrier and its system was quite good. It had more cachet than CompuServe or aol, and was more used by business people. Then ibm decided to get out of the business and sold the email service to at&t, which was struggling to build a decent sized isp. There were two sides, one for consumers, the other for business oriented people. The ibm addresses were maintained for a few years then changed to at&t. The business oriented addresses were attworld.net, but then changed to att.biz, though the former is still valid.
-er