You say return"s".  How many returns did you mail and did you mail them in one big envelope?  If you mailed them separately and they are all showing up late, you might have a tougher problem getting out of the penalty than if you were dealing with one return.  This is a good example of why I don't like mailing returns for clients.  Let the client mail and at least you are off the hook.  I'm not sure if there was a reason you had to paper file the returns, but this is a good example of why e-filing is the preferable route to go with anything that can possibly go that route.  In a worse case scenario, at least e-filing an extension to allow some room for error couldn't hurt.