If you received it directly from IRS then its most likely legitimate. If it has a phone number to call I would first make sure the phone number matches what the IRS actually provides on its web-site.
Copied from the IRS web-site:
https://www.irs.gov/individuals/understanding-your-5071c-letter
We received a federal income tax return with your name and taxpayer identification number but we must verify your identity to process it accurately. The contact information below is only for taxpayers who received Letter 5071C.What you need to doHow to contact us online:
- Use our secure Identity Verification Service. It’s quick and secure.
- You can opt-out of the Identity Verification Service at any time and call the toll-free number provided in the 5071C letter.
- If you answer a question incorrectly, you can call the toll-free number in the 5071C letter.
How to contact us by phone:
- Call the toll-free IRS Identity Verification telephone number in the 5071C letter.
- Have a copy of the 5071C letter you received, a copy of your prior year tax return (if you filed one) and your most recently filed tax return (if you filed one), as well as any supporting documentation for each year's return (such as Forms W-2, 1099, 1040 Schedule C, or 1040 Schedule F) when you call.
We’ll ask you a series of questions about your identity and tax return history.
If we are unable to verify your identity over the phone or online, we may ask you to schedule an appointment at your local IRS office to verify your identity in person.
The toll-free number is for identity verification only. No other tax-related information, including refund status, is available.
