Research In Motion still doesn't know why its BlackBerry service went down for several hours on Monday.

"RIM is continuing to investigate the exact cause" of the outage, the company said in a statement Tuesday. Late Monday it apologised for any inconvenience caused by the incident, which left customers throughout North America without current email for about three hours starting around 3:30 pm Eastern Time.

It was the second major outage in less than a year for the popular mobile data service, on which about 12 million subscribers depended at the beginning of last December. The previous problem, which occurred last April, was caused by a minor software upgrade that went awry, followed by a failed switchover to a backup system, according to RIM. The company said soon afterwards that it had identified "certain aspects of its testing, monitoring and recovery processes that will be enhanced" as a result of the failure.

BlackBerry email traverses a complex infrastructure involving mobile operator networks, RIM's network operations centre and BlackBerry Enterprise Servers within companies that use the service. It pushes messages from enterprise email systems, including Microsoft Exchange and IBM Lotus Notes, out to the popular BlackBerry devices.

The system is getting yet more complicated as RIM adds third-party services to appeal to consumers, said Albert Lin, an analyst at investment bank Sooner Cap. As the company tries to keep up with rapid growth in its subscribers - last year's fiscal third quarter saw a net gain of 1.65 million - these types of glitches are to be expected, Lin said.

"It's hard to really expect any major service provider to be 100 percent reliable," Lin said. Although enterprises now have more push email alternatives than they did when the BlackBerry debuted in 1999, those competitors, such as Visto and Motorola's Good Technology system, aren't significantly more dependable, he said.

"When it comes to reliable push email ... it's still hard to find a solution that works better than BlackBerry," Lin said.