I also didn’t get their point but actually, using their API actually causes the mails to deliver as normal and otherwise the mails end up in spam of specifically one service provider i.e. microsoft.
I don’t really trust microsoft when it comes to reliable spam protection but when there is a majority of users using their service, I thought that maybe it’s worth investing upon some solution to figure this out.
also, the same happens even if the SMTP provider is changed. I’ve replicated this same behaviour with mailgun & elasticemail as senders. Hosting my own SMTP doesn’t seems like a Good idea either.
I’m still waiting for them to diagnose the mail headers and come up with some findings.