I’v installed PHPList on a fresh VPS with Ubuntu 14.04.2 LTS.
I was trying to send emails and I was getting the following error Could not instantiate mail function.

on a fresh VPS with…

• PHP5 IMAP
• Sendmail

sudo apt-get install php5-imap

and

sudo apt-get install sendmail-bin

Probably only this last one was enough.
It worked!
Hooray!