NEW YORK (CNNMoney.com) - The tax deadline has come and gone... and you didn't give filing a single thought.

More than 420,000 Americans faced IRS fines for delinquent tax returns in 2005. While that's a small number compared to the total number of returns filed - 133 million last year - it's still a whole lot of deadbeats for the IRS to chase down.

Over 400,000 Americans turned in delinquent individual income tax returns in 2005.

And the agency collected almost $23 million from delinquent taxpayers last year, not including what had already been withheld.

If you've let your returns slide this year and you owe money, the IRS will right off the bat levy a failure-to-file civil penalty. The IRS will hit delinquents each month with 5 percent of the tax not paid by the due date, but not more than 25 percent in total. So if you owed the agency $5,000, you'd face $250 a month, or $1,250 after 5 months of delinquency.

The IRS urges everyone to file, even if they think they don't owe money. "If you owed nothing, there would be nothing to assess penalties against," said an IRS spokesperson. "But there's always the possibility that the taxpayer made a mistake, and then they'd be subject to interest penalties and tax."

"Even if there are no taxes owed, it's still against the law not to file if you have a filing requirement," a spokesperson said. In most cases, if people make over $8,200 a year, they're required to file.

If you're required to and didn't file, the IRS will mail you a notice stating that the your return is still due.

If you still haven't filed after 60 days (that's Saturday, June 17th, for the delinquents in the audience), the agency institutes a minimum penalty - the smaller of either $100 or 100 percent of the unpaid tax. So if you'd had been facing only the 5 percent penalty before, you could face a jump in the penalty after 60 days.

Still, even if you're very late, there's an outside chance that you could get what the IRS calls a "reasonable cause abatement." In this case, the IRS will waive all the penalties. Just what constitutes a good reason, the IRS won't say.

"It's very, very circumstantial, always on a case-by-case basis," said a spokesperson. "If you think you've got a good reason that you weren't able to meet the deadline, explain it. Send us a letter."

And it's not a particularly good year to be an IRS delinquent - the agency has stepped up its enforcement efforts on all fronts, from individual income collection to corporations using tax shelters.

During 2005, the Justice Department's Tax Division sought prosecution against 1,256 defendants for tax crimes, up over 43 percent from 2001. And while most of the charges are high-level avoidance schemes, some of those people were taken to the courtroom over their simple failure to file.

The Tax Division also forces fraudulent tax preparers to halt their operations, and it looks as if some of those preparers are really trying to pull a fast one: among the schemes stopped last year was a return "purporting to pay employees in commodities such as milk."

"The vast majority of Americans pay their taxes honestly and accurately," said IRS Commissioner Mark W. Everson. "With the help of the Justice Department, the IRS holds those who don't accountable."

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