IANAL, but looks like it's addressed on page 9 here: https://www.irs.gov/pub/irs-irbs/irb97-13.pdf https://www.irs.gov/pub/irs-irbs/irb97-13.pdf I'd be shocked

IANAL, but looks like it's addressed on page 9 here: https://www.irs.gov/pub/irs-irbs/irb97-13.pdf https://www.irs.gov/pub/irs-irbs/irb97-13.pdf I'd be shocked if it wasn't, since invoicing purely by e-mail is pretty common these days.