Cannot send email via contact form using Web Presence Builder: Your message could not be sent due to an internal error

Symptoms

1. Cannot send email via contact form using Web Presence Builder:

   CONFIG_TEXT: Your message could not be sent due to an internal error. Please contact the website administrator by other means to notify about this issue

   But in fact, the email is sent and received successfully.

2. Error in strace log:

   CONFIG_TEXT: PHP Fatal error: Uncaught exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail. mail(/var/tmp/php.mail.log): failed to open stream: Permission denied' in /var/www/vhosts/example.com/httpdocs/includes/Zend/Mail/Transport/Sendmail.php:

Cause

Incorrect permissions on/var/tmp/php.mail.logfile.

Resolution

Note: if you don't have root access to Plesk server via SSH, contact your hosting provider regarding the issue

1. Login to Plesk server over SSH
2. Set the correct permissions on/var/tmp/php.mail.logfile:

   # chmod 664 /var/tmp/php.mail.log
   # ls -la /var/tmp/php.mail.log
   -rw-rw-r-- 1 root root 104306 Sep 5 22:47 /var/tmp/php.mail.log