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

Symptoms

Cannot send email via contact form using Web Presence Builder. The following error appears in browser:

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, an email is sent and received successfully.

In strace log, the following error appears:

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.log file.

Resolution

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

   # 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