Sparkpost SMTP: emails not being delivered to

I have been hitting my head over this issue for 3 years now.

We are a small ecommerce store using Sparkpost API to send email.

We have problems sending email to outlook / hotmail / Microsoft365 mail based email addresses.

Their DNS MX record is:

  • We have correct SFP, DKIM, DMARC setup
  • Not on any known blacklists
  • Dedicated IP address
  • We applied and are on SNDS and JMRP programs
  • Emails are marked as transactional

But our emails are still landing into spam or don’t get delivered to the customer.

Only way to send the customer an email is from Gmail based on the same domain.

So I assume this has something to do with Sparkpost IP or headers they attach.

Any ideas how we can resolve this?

Sparkpost has not been helpful. They are just saying to send more volume(which is not possible for a small store) and look at email templates (which I’m in process of simplifying).