dmertz is correct that you have to pay the additional tax due and any interest in any case (but see below), but you may be able to get an abatement of the penalty (as the link he gave you shows). After all, It was a check from a "life insurance company" and it was your "ex-husband" with whom, presumably, you did not frequently communicate. However, you may feel awkward in how you would word such a response. In this case, you might turn to a local Enrolled Agent or CPA to represent you to the IRS, which would mean writing the letter to the IRS in terminology that the IRS understands. You can find local Enrolled Agents at https://taxexperts.naea.org/ and use your search engine to find local CPAs.
macuser_22 does open another issue: it's possible that not all of the annuity income was taxable. Yes, sometimes that will be reflected in box 2a on the 1099-R, but all too often, the annuity provider is not aware of whether or not the taxable amount should be less than the total distribution.
The issue is that if your ex-husband made any after-tax contributions to the annuity while he was alive, then when these contributions (called "cost" or "basis") are returned to you (according to a formula), you do not report the amount of basis returned as taxable income.
The reason that this is important is because any amount that is non-taxable reduces the tax you owe as well as the interest, whereas the first issue is just trying to avoid paying the penalty.
Sadly, I have had clients who were widows who had no idea whether or not their late husbands made after-tax contributions to their annuity. You can ask the annuity company if they know (but they may not for various reasons) or his employer to see if they know, but this is perhaps another reason to ask for local professional tax help, because the local Enrolled Agents and CPAs will know what questions to ask the annuity provider and the employer, as well as what to look for in your late husband's papers.
Good luck.
