E-filing for Individual returns (Form 1040) stopped on November 17th and won't open again until mid-to-late January.  So those should probably be mailed.

E-filing for Business returns (Corporations, Partnerships, etc.) is still open until mid-to-late December.  So those can still be e-filed.

As Marc mentioned, extensions (or lack thereof) or any balance due does not affect whether it can be e-filed or not.