email sent via Opencart blocked by some ISPs (and gmail) as spam

Hi, I am not sure if this is an issue related specifically to OpenCart, or the way my email is configured via Cpanel. I am posting here with the hope that someone may have dealt with this issue before, and can help me identify the source of the problem so that I can seek appropriate help. I have a problem with my OpenCart order confirmation emails being blocked as spam by some ISPs. Example: I received an email from a customer, which I read on Thunderbird, replied to, and the customer replied to my reply. No problems. 1 hour later the same customer placed an order, using the same email address, and the order confirmation email sent out by Opencart immediately bounced back, marked as spam. I have both SPF and DKIN configured correctly on my mail server (VPS), and emails sent via Thunderbird show a PASS when testing with Port25 authentication testing service. In fact, I have had an ongoing issue for nearly a year (since I moved to OpenCart from a 3rd party eCommerce product). I have never had issues with my mail being rejected, but since using Opencart I have had many order (transaction) emails sent by OpenCart, blocked as spam (I do not use my email address or IP for newsletters). Could my issue be related to the problem described (and offered a fix for) in this post: /http://www.expertsos.net/blog/opencart-fixing-notice-error-rcpt-to-not-accepted-from-server/ ?