This sounds like it’s an issue with your customers mail providers rather than your Mendix app. Mendix is sending the email to SendInBlue, which is in turn sending it to the customer, where their mail provider is marking it as spam.
Are you using things like a genuine email addresses in the from field, the customers details in the body of the email, no “spammy” text? If you can get the customer to interact with your email once, it less likely to be marked as spam in the future.
It’s a bit of dark art, and unfortunately delivery is not something you can guarantee.