interest and penalties - safe harbor

[Mike Dymski]

I received an IRS tax notice with interest and penalties that related to the balance due with the filing of the extended return in October.  Tax withholdings were well over 110% of the prior year tax liability; so, I am comfortable that the penalties will go away.  Does the interest go away as well or is that due regardless?  Intuitively, it seems like there should be no interest due if no tax was due based on the 110% safe harbor but I don't know what I don't know.