MP3.com founder Michael Robertson is on the receiving end of another lawsuit - from EMI - this time against his MP3tunes.com and Sideload.com services.

Robertson told Reuters it was retaliation for a suit his Sideload.com service had filed against EMI in September.

MP3Tunes is a service that enables the user to back up their music collection and access it anywhere. Multiple concurrent access is prohibited - so it's hard to imagine how it could be construed as infringing copyright. In fact, it's just the sort of service recording sound owners should be offering as a value-add to physical offerings.

The legality of Sideload.com is more dubious. This additional service the music stored in MP3Tunes' "lockers" and allows subscribers to grab a copy and drop them into their own locker. Sideload ducks the copyright issue.

"It's possible some of the tracks may be unauthorised," Robertson told a local paper when the service launched. "But the difference between Sideload and [the original] Napster is that we're simply a search engine; we're no different than Google."

As with any technology, what's "illegal" today could simply be viewed as "not licensed... yet".

It's seven years since Robertson's MP3.com paid over $200m in damages for operating a similar service. A judge ruled that MyMP3.com breached mechanical copyright by allowing users to upload legitimately-acquired CDs and stream them over the internet. Robertson later sold MP3.com to Universal Music for over $370m.

But with almost half of all BitTorrent transfers now encrypted, and impossible to track - you might think the music business would see services such as Sideload.com as a revenue opportunity.

After all, you can't count something you can't see. ®