Fatal error: Uncaught exception 'Zend_Mail_Transport_Exception' with message 'Unable to send mail' in /home/content/v/o/l/vol4ikman/html/concrete/libraries/3rdparty/Zend/Mail/Transport/Sendmail.php:105 Stack trace: #0 /home/content/v/o/l/vol4ikman/html/concrete/libraries/3rdparty/Zend/Mail/Transport/Abstract.php(348): Zend_Mail_Transport_Sendmail->_sendMail() #1 /home/content/v/o/l/vol4ikman/html/concrete/libraries/3rdparty/Zend/Mail.php(973): Zend_Mail_Transport_Abstract->send(Object(Zend_Mail)) #2 /home/content/v/o/l/vol4ikman/html/blocks/external_form/forms/controllers/contact_form.php(148): Zend_Mail->send() #3 [internal function]: ContactFormExternalFormBlockController->action_submit_form() #4 /home/content/v/o/l/vol4ikman/html/concrete/libraries/controller.php(190): call_user_func_array(Array, Array) #5 /home/content/v/o/l/vol4ikman/html/concrete/blocks/external_form/controller.php(67): Controller->runTask('action_submit_f...', Array) #6 [internal function]: ExternalFormBlockController->__call('action_submit_f...', in /home/content/v/o/l/vol4ikman/html/concrete/libraries/3rdparty/Zend/Mail/Transport/Sendmail.php on line 105

$to = array("myemail1@mail.com", "myemail2@mail.com");

$to = array("myemail1@mail.com", "myemail2@mail.com");

<form action="<?php echo $this->action('submit_form')?>" 
   method="post" 
   onsubmit="return (checkField(this.name, 'Please enter your name\n(2-100 characters)', 70) && 
   checkEmail(this.email, 'Please enter your valid email address\n(8-100 characters)', 100) && 
   checkField(this.ccmCaptchaCode, 'Please enter image validation code\n(5 characters)', 5) && 
   checkField(this.message, 'Please enter your message\n(2-3000 characters)', 3000));" >

<div id="contact_form">
<form action="<?php echo $this->action('submit_form')?>" 
   method="post" 
   onsubmit="return (checkField(this.name, 'Please enter your name\n(2-100 characters)', 70) && 
   checkEmail(this.email, 'Please enter your valid email address\n(8-100 characters)', 100) && 
   checkField(this.ccmCaptchaCode, 'Please enter image validation code\n(5 characters)', 5) && 
   checkField(this.message, 'Please enter your message\n(2-3000 characters)', 3000));" >
<div>   
<?php
//print $form->label('name', t('Name: '));
//print $form->text('name', $name, array('maxlength' => 70, 'size' => 40, 'onblur' => "check('name');"));
?>
<label for="name" class="required">Name:</label> 
<input type="text" id="name" name="name" value="" maxlength="70" size="40" onblur="check('name');" />
</div>

$error_msg = '<div id="error_msg">If you are experiencing problems submitting the form please send an email with your query to <b>'.$to.'</b>.</div><hr />';
      $no_error_msg = '<div id="noerror">Thank you for your inquiry.</div><hr />';
      if (!$ip->check()) {
         $e->add($ip->getErrorMessage());
      }      
      if (!$captcha->check()) {
         $e->add(t('Incorrect image validation code. Please check the image and re-enter the code.'));
         $_REQUEST['ccmCaptchaCode']='';
      }
      if (strlen($name) < 2) {
         $e->add(t('Name must be at least %s characters long.', 2));
      }
      if (strlen($name) > 70) {
         $e->add(t('Name cannot be more than %s characters long.', 70));
      }

$to = "YOUR_EMAIL_ADDRESS";

checkField(this.ccmCaptchaCode, 'Please enter image validation code\n(5 characters)', 5) &&

checkField(this.ccmCaptchaCode, 'Please enter image validation code\n(6 characters)', 6) &&

.....
Loader::library('3rdparty/Zend/Mail');
$config = array('auth' => 'plain', 
'username' => 'myUserName',
'password' => 'myPassWord');
$transport = new Zend_Mail_Transport_Smtp('mx.domain.tld', $config);
$zm = new Zend_Mail(APP_CHARSET);
Zend_Mail::setDefaultTransport($transport);
$zm->setSubject($subject);
$zm->setBodyText($txt_message);
//$zm->setBodyHtml($html_message);
$zm->addTo($to);
$zm->setFrom($email);
$zm->send();
.....

...
//Zend_Mail::setDefaultTransport($tr);
...
$zm->send($transport);
...