Email error

This error started popping up a few days ago. 

ERROR - 2018-01-20 10:29:06 --> Severity: Warning --> mail(): Could not execute mail delivery program '/usr/sbin/sendmail -t -i ' /var/www/system/libraries/Email.php 1894

I am running the most current version of CI on an updated Ubuntu machine. I put in some logging right before this line and what's odd is the emails that it says aren't sending are sending, i.e. this message is a false positive.

Thoughts?

Joe