I was wondering if last date of submission of US taxes for 2017 means that tax should reach IRS by April 18 or should be posted to IRS by April 18. Any references would be helpful.
According to USPS:
Federal Tax Deadlines. If your tax return is postmarked by April 18, 2017 or October 16, 2017 (extension), the IRS considers it filed on time.
According to the IRS:
Your return is considered filed on time if the envelope is properly addressed, postmarked, and deposited in the mail by the due date.
So on top of postmarking it, it has to be properly addressed as well. So if it gets returned to you because the address was wrong, then you might be penalized.