It can't be e-filed. the IRS wants your actual signature and actual copies of form W-2 because you mailed in your return.
Usually, Form 3531 has an address in the upper left corner to send the form. Lacking that address, I'd suggest sending it to the address specified here
https://www.irs.gov/filing/where-to-file-addresses-for-taxpayers-and-tax-professionals-filing-form-1...
use address for not enclosing payment. be patient mailed in forms take months to process
