I believe that you can only efile the current year.  All previous years must be mailed.

No you can efile 2017, 2016 and 2015 done of course in that corresponding year of the software.  IRS doesn't accept anything beyond that.

"I believe that you can only efile the current year"

I believe that your believing is not quite accurate.  If not, the 2015 and 2016 returns I successfully e-filed a couple of weeks ago were mere figments of my imagination.

Heads up, if you're filing states to make sure they also allow prior year e-filing.  Some like to be different. For example, Kentucky.  Most states conform, the heads up is just an item to verify in case you run into any errors during transmission.

Good point.

thank you