Manual talk:$wgEnableEmail

SELinux causes Mail to fail silently
If you enable email, and it's not working, you might want to check the Security Enhanced Linux (SELinux) settings in RHEL6, Scientific Linux, etc. to find out if the webserver is allowed to send email. If it's not, you'll get failures but without any helpful information on why mail is not working. httpd_can_sendmail --> off Allow Apache to send email, using -P to persist the setting across reboots:

Testing sending emails
Hi there. I just installed 1.26.2, and I suspect that I can't send email on my server (getting "Mailer returned: Unknown error in PHP's mail function." when creating a user). Excuse my ignorance, but how do I test this hypothesis please? Would be great if this were documented, or even better if mw-config/index.php auto-detected this problem, with the help of the user, if necessary. Thanks. 146.198.223.71 20:19, 18 May 2016 (UTC)
 * Solved and documented, thanks. 146.198.223.71 23:51, 21 May 2016 (UTC)