Can't send emails via SMTP (quote form)

Can't send emails via SMTP (quote form)

Hello,

My website is khantransport.com (built on HTML) hosted on Linode (Ubuntu 16.04.1 LTS). I have a quote form on the website, which works like a contact form. All the details of the form are to sent to one single email address all the time. However, I am having troubles with the mail function.

When I am running my site locally, upon clicking 'SEND' on the quote form, the email is going through and I receive the email on my zoho account (haris@khantransport.com). However, on the live version of the site the email is not going through. I used the following guide for smtp settings:

1). Installed SMTP 
sudo apt-get install ssmtp

2). ssmtp.conf configuration
File Location: /etc/ssmtp/ssmtp.conf

mailhub=smtp.zoho.com:587
UseSTARTTLS=YES
AuthUser=haris@khantransport.com
AuthPass=<YOUR-PASSWORD>

3). php.ini configuration
File Location: /etc/php/7.0/apache2/php.ini

sendmail_path = /usr/sbin/ssmtp -t

Also, when I run the following command: /usr/sbin/sendmail -V 
This is what i get: sSMTP 2.64 (Not sendmail at all)
I also get this error for test mail:  sendmail: 553 Relaying disallowed as "root"