Thread:Project:Support desk/Unknown error in PHP's mail() function

MediaWiki: 1.23.5 PHP: 5.5.9-1ubuntu4.4 (apache2handler) MySQL: 5.5.40-0ubuntu0.14.04.1

I installed a new wiki. It is working fine other than it fails at sending email. For example when I ask it to send a confirmation email I get the following error:

"4MI Wiki could not send your confirmation mail. Please check your email address for invalid characters. Mailer returned: Unknown error in PHP's mail function."

Or when I try to create a new account and have a random password sent to the user I get this error: "Account creation error Error sending mail: Unknown error in PHP's mail function."