I use SparkPost to send email but The email that I send it goes in spam. The send email that I use is :
sandbox@sparkpostbox.com
maybe it's for this that the email goes in spam.
The SparkPost sandbox is available for everyone to send 50 emails. It is mainly to make sure your account is functioning properly. Because of this, the sparkpostbox.com domain doesn't will probably never have a great sending reputation. You should setup and verify a sending domain for better inbox placement.