Check your letter. Its probably on an unpaid balance due rather than a new assessment. (10 years for collection of balance due rather than assessment of additional tax IIRC.)
lite reading!
https://www.irs.gov/irm/part25/irm_25-006-001r
25.6.1.2 (10-01-2001)
What is a Statute of Limitation
A statute of limitation is a time period established by law to review, analyze and resolve taxpayer and/or IRS tax related issues.
The Internal Revenue Code (IRC) requires that the Internal Revenue Service (IRS) will assess, refund, credit, and collect taxes within specific time limits. These limits are known as the Statutes of Limitations. When they expire, the IRS can no longer assess additional tax, allow a claim for refund by the taxpayer, or take collection action. The determination of Statute expiration differs for Assessment, Refund, and Collection.
“General Assessment Period
The general rule is that the assessment of tax must be made within three years after the return is filed. See IRC ยง 6501(a).”
25.6.1.9.5.3 (09-20-2016)
25% Omission
The tax may be assessed within six years after the original return was filed (IRC Section 6501(e)), if the taxpayer omits:
More than 25% of the gross income reported on original Form 1040, 1041, 1042,1120, 990-C, 990-T or 990-PF (IRC Section 6501(e)(1)(A)(i))
More than $5,000 in income attributable to foreign financial assets required to be reported on Form 8938 (disregarding the dollar reporting threshold), regardless of whether or not the specified foreign financial assets are reported on Form 8938 or another IRS form per IRC Section 6501(e)(1)(A)(ii) (the Form 8938, Statement of Foreign Financial Assets reporting requirement applies only to tax years beginning after March 18, 2010)
More than 25% of tax on original Form 720 and Form 5330 for excise tax (IRC Section 6501(e)(3))
Includable items in excess of 25% of the gross estate on original Form 706 (IRC Section 6501(e)(2))
Gifts in excess of 25% of the total gifts on original Form 709 (IRC Section 6501(e)(2))...
etc.