Updated at 3:25 p.m. PT after service was restored.

Hot iPhone app Mailbox suffered a major outage today, but the app is now back online for most users.

Orchestra CEO Gentry Underwood said the exact issue that crashed the application hasn’t quite been determined, but it is figuring it out and will stop new users from getting in for now.

“We hit some limits that we need to spend more time understanding, so we’re going go keep the reservation system paused until we know more,” Underwood told VentureBeat.

Prior to the outage we spoke to Underwood, who said his 15-person team was doing everything they could to address the problem.

“We’re busting our asses to get it back up,” Orchestra CEO Gentry Underwood told us while the outage was occurring earlier today. “The system is down for everyone right now. All of our team is working on it.”

Mailbox attracted a lot of attention when it debuted last week to astounding demand. The attention mostly came from the promise that Mailbox could help an average Gmail user manage their email better than any other iPhone app. It offers stripped-down email messages to make them easier to read, a “snooze button” to push back a message’s placement, and smart push notifications.

When we first wrote about it last week, more than 380,000 people were waiting to use Mailbox. Now the queue sits at about 800,000 people. “Throttling new users as they are added to the system has been incredibly powerful at keeping it from falling over,” Underwood said.

When the app launched, some folks who were placed in the long queue accused the makers of doing the slow rollout to generate buzz. But Underwood said the purpose of the line was to create a controlled flow of users and make a crash less likely. Turns out even with the line, the system was still vulnerable.

“Even with rate limiting in place, we can’t foresee every problem,” Underwood said. “The team has been working 24-hour shifts since launch.”

Mailbox’s Twitter account responded furiously to user complaints and questions, trying to keep disgruntled users at ease.