Updated at 11 p.m.: Revised to reflect that the shutdown has begun.

Don't worry: You've still got mail.

But with President Donald Trump and Congress at odds over a Senate-approved funding bill, federal agencies began a partial government shutdown at midnight Friday.

The discord affects nine of 15 Cabinet-level departments, numerous agencies and 800,000 government workers, The Associated Press reported. More than 400,000 of those employees will be compelled to keep working without pay — including employees of the National Weather Service.

Essential services won't be interrupted. That includes mail delivery, Medicare and Medicaid reimbursements, Social Security checks and military and law enforcement operations — including border security, which is at the root of the funding conflict.

About three-fourths of government services, already funded by existing legislation, will also continue, as will the Mueller investigation.

Here's a list of some of the agencies that will and won't be affected.

What could be affected

National parks could close or, as with the shutdown earlier this year, be open to visitors but with limited staff and some park facilities closed.

The State Department will still issue passports and visas, but could stop doing so if those services are conducted at facilities run by agencies that have shut down.

Inspections of hazardous sites and food products could diminish with limited operations by the Environmental Protection Agency and the Food and Drug Administration, according to Vox.

Small Business Administration loans will go unprocessed, and Federal Housing Administration loans will be delayed, according to nj.com.

The Internal Revenue Service plans to furlough 52,000 workers, since tax season is not imminent. And nearly all staff at NASA — which has thousands of Texas workers — will be sent home as well, according to Space.com.

What won't be affected

Social Security, Medicare and Medicaid operations will not be interrupted, though as Vox reported, new applicants may face delays.

The U.S. Postal Service, an apparently unstoppable independent agency undaunted by rain, sleet, snow or partial government shutdowns, will continue operations, as will "essential" services like air-traffic controllers, TSA agents and the Federal Emergency Management Agency.

With funding already secured, the Department of Veterans Affairs is unaffected, as are veteran disability pay and GI benefits, which, according to Military.com, are funded by previous legislation.

Food stamps and subsidized lunches will continue to be provided, though an extended shutdown could change that; the U.S. Department of Agriculture in the past has had only limited funding without new appropriations.

Active-duty military members are exempt from shutdown furloughs, though their pay could be delayed by an extended shutdown. So, too, for employees of U.S. Customs and Border Patrol and Forest Service firefighters.

Robert Mueller's Russia investigation, with its own source of funding, will go on, according to CNN.

And many will be relieved to know that the North American Aerospace Defense Command (NORAD) will continue its 63-year tradition of monitoring Santa Claus' movements on Christmas Eve.

"Military personnel who conduct NORAD Tracks Santa are supported by approximately 1,500 volunteers who make the program possible each and every year," CNN reported the agency as saying.

With information from The Associated Press, Vox and nj.com.