NEW YORK, Sept. 2 (UPI) -- Google said Wednesday that a capacity miscalculation was responsible for Tuesday's nearly two-hour outage affecting Google Mail.

A classic cascade in which servers become overwhelmed with traffic left millions of GMail customers out of luck between 3:30 p.m. EDT and 5:30 p.m. EDT, CNET News reported Wednesday.

Google said the problem began when several servers were taken offline for maintenance.

At the same time, Google says it had to make some changes to the routers that direct GMail traffic to servers in hopes of improving reliability.

Those changes backfired, Google says.

Google was able to fix the outage by allocating traffic across the rest of its network.

Google says in the future it will focus on making sure that request routers have sufficient headroom to handle future spikes in demand.