The IRS entered the first day of tax season with 5 million pieces of unopened mail, CBS News has confirmed -- lingering effects of the government shutdown that could take the agency more than a year to recover from. Since in-person IRS help centers were closed, taxpayers turned to the postal system, and the IRS went from getting 200,000 pieces of mail per day to 700,000.

That's not the only backlog the agency faces. Some IRS workers quit during the shutdown, and the agency has delayed hiring thousands of temporary workers, which it typically does before tax season. Michelle Harris, an IRS employee, told CBS News she expected "massive catch up at this point."

Here's what this means for tax filers.

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"Hundred-meter dash"

For many tax pros, the most salient factor of the shutdown isn't that it's over -- it's that another one may be coming in just three weeks.

"Now, it's like the hundred-meter dash -- we have 21 days to get it done, and then who knows?" said Mark Steber, Jackson Hewitt's chief tax officer. Jackson Hewitt has been accepting returns for the last two weeks, but fewer people than normal have been filing, Steber said. "I would attribute that 100 percent to people hearing the government was shut down."

Steber expects a surge in tax filing over the next three weeks, above and beyond the 50 million returns that were filed during this time period last year.

Will your refund be bigger?

On the whole, Americans paid less in taxes last year, but that doesn't mean individual refunds will be bigger.

The Government Accountability Office estimated this summer that about 21 percent of taxpayers weren't withholding enough from their paychecks and would end up owing at tax time, up from 18 percent if the tax cuts hadn't been passed. That translates into 4 million more Americans who'll find out they owe taxes come filing season.

People who'll pay more taxes than before include those in costly, high-tax states like New York and California, UBS analysts said recently.

Ben Gilbert, a construction executive based in Massachusetts, expects his filing to be "faster and easier" this year -- but not necessarily cheaper. "I'm told to expect a negative effect because of my personal situation and that most people should see an improvement," he said.

The IRS in December warned taxpayers they may owe money, and then it followed up by dropping some penalties for underpayments.

But experts disagree. "There's been a lot of speculation," said Steber. "There's no data that I've seen -- ours, competitors' -- that have an answer."

He added: "I do think more taxpayers will have a lower tax liability. Does that mean a bigger refund? That depends on what they did, or how their employers handled withholding."

In a typical year, about three-quarters of U.S. taxpayers receive refunds. For lower-income households, in particular, a tax refund is their biggest cash infusion of the year.

Will your refund be delayed?

If you're claiming the Earned Income Tax Credit or Child Tax Credit, as about 30 million taxpayers do, then yes. For the last few years, the U.S. government has held these refunds until mid-February as part of a crackdown on fraud, and that's expected to continue this year.

For other taxpayers, the answer is less clear.

"People do have questions about how the shutdown impacts their refunds, and it depends on the taxpayer's specific situation," said Alison Flores, principal tax analyst at H&R Block.

In a normal tax year, most refunds are processed in 21 days or less, the IRS says.

Phone help?

It's not yet clear what the IRS' capacity is to answer questions via phone. The agency answered 95 million phone calls during last year's tax season, but only about 40 percent of callers were eventually put through to a real person, the Associated Press reported.

The IRS didn't immediately respond to a request for comment.

Aimee Picchi contributed reporting.