Customer tracking Emails ending up in Junk / not being received by customers

I'm looking for someone to create some sort of system where I can upload a csv file with the customer names, emails and the order tracking numbers and it will send an email to the customers with their order tracking information. I'm currently using a free SMTP relay sever to send out my customer tracking emails for the customers orders. What I do is upload a csv file with the customer names, emails and the order tracking numbers to the site and send it out. The problem I'm getting is that more than 50% of these emails are going into junk/ not be received by the customer. spam check the email contents, the server, email subject etc, but can't work out why they're not being received so looking for a new system.