Email notifications failing in Elgg 2.3.2

I am fairly new to Elgg, but not to Linux and PHP.  My server setup is:

Elgg version :     Release - 2.3.2, Version - 2016092300
Apache/2.4.18 (Ubuntu)
PHP 7.0.15-0ubuntu0.16.04.4
ssmtp v2.64

When anyone sends requests a password reset, no email is sent. The following is from /var/log/apache2/error.log:

ssmtp: RCPT TO:<toaddress@example.com> (550 Sender verify failed)
[Tue May 23 08:16:50.151803 2017] [:error] [pid 18096] [client xxx.xxx.139.246:54768] ERROR: Unable to send mail: Unknown error, referer: https://xxx.xxx.org/social/

ssmtp has been verified to work successfully with the following code:

<?php
$address = "toaddress@example.com";

$subject = 'Test email.';

$body = 'If you can read this, your email is working.';

echo "Attempting to email $address...<br />\n";

if (mail($address, $subject, $body)) {
        echo 'SUCCESS!  PHP successfully delivered email to your MTA.  If you don\'t see the email in your inbox in a few minutes, there is a problem with your MTA.';
} else {
        echo 'ERROR!  PHP could not deliver email to your MTA.  Check that your PHP settings are correct for your MTA and your MTA will deliver email.';
}

I have inserted logging statements in the elgg/engine/lib/notification.php file and the email from and to addresses, and message body are being created correctly, I think.

It gets to the function "_elgg_notify_user" and seems to fail in the following function call (line 423).

                    try {
                        $result[$guid][$method] = call_user_func(
                            $handler,
                            $from ? get_entity($from) : null,
                            get_entity($guid),
                            $subject,
                            $message,
                            $params
                        );
                    } catch (Exception $e) {
                        error_log($e->getMessage());
                    }

I am not that familiar with Zend\Mail which I am assuming is being called in the above code.  Is there configuration info on Zend\Mail transport options somewhere?  If no one has any ideas, I may replace the above call with a straight PHP mail function call to get this working.  I suspect that would break other notification methods, but I would rather have email working by itself than nothing.