When a taxpayer comes to you and tells you that they have multiple years of unfiled tax returns, before you start filing, fill out the 8821 and get a complete report for all the years that the taxpayer has not filed. This request will not alert the IRS to go after the client.
Generally, the IRS is only interested in the last 6 years of tax returns according to their policy statement P-5-133. Be aware that there is no civil statue on unfiled returns, but the criminal statue is 6 years.
Do not mail all the back years in the same envelope. With each return, file a letter of reasonable cause for abatement of penalties. The chances are great that each of the returns you file will go to a different examiner. So if you have 6 returns, you have 6 chances the examiner might buy your reasonable cause for abatement of penalties. Make sure you pay a small amount of money on each return, even if no money is owed. This way the IRS cannot say they did not get the return. On the back of the check you send in will have the IRS notation for what tax return this payment is to be attributed. Therefore proof of filing.