Costs Billions. How believable is this?

The Washington Post reports that

Time wasted deleting junk e-mail costs American businesses nearly $21.6 billion a year, according to a new study from the University of Maryland.

A telephone-based survey of adults who use the Internet found that more than three-quarters receive spam daily. The average spam messages per day is 18.5 and the average time spent per day deleting them is 2.8 minutes.

So 2.8 minutes/day is worth $21.6 billion a year. If people work a full 8 hour day, 2.8 minutes is about 0.6% of the normal work day. That must mean that the total value of work done in the country is $3.6 trillion (21.6/0.006).

The Bureau of Economic Analysis: National Economic Accounts reports, if I understand the figures correctly, that the GDP for 2004 was about $12 trillion. Are these two figures consistent? Is labor really only 25% of the GDP? Perhaps spam is wasting much more than $21.6 billion.

On the other hand, does it really make sense to attempt to quantify the time wasted by a 2.8 detour from work. How much money is wasted by bathroom breaks? Would that much more work really have been accomplished if spam didn't exist?