u/CharacterAnt5866

My husband got a letter in the mail from the IRS that he has an account balance of $4,235 from the taxable year 2021???. This is super confusing - why is this happening? He has a normal 9-5 job, filed his taxes and I believe got a tax return that year. Why all of the sudden he actually owes this amount? Is there a way they could be wrong and we can contest this?