When sending any emails, including SendPulse, OneBox only prepares emails and sends
mail service which, and is engaged in sending letters.
The SendPulse integration says:
- API SendPulse will be used
In order to understand why the letter got into spam, you need to study the headers of this letter.
To check the settings (including DNS) here is a good resource
https://www.mail-tester.com/Send an email to the address and view the report on what is good and what needs to be adjusted.