We're sending mails using c# Mail library, we need to send specific headers to track them through external providers. We've tried some months ago and it worked, but now trying again before deploying in production it doesn't work.
We're using corporative Office365 (smtp.office365.com) as smtp server and we can't see anything on exchange console that points to know what is the problem. The generated message (as seen on "sent" folder) has the headers but the received message no.
We try on a common gmail account (smtp.gmail.com) and it works fine, the mail headers arrive to the receiver.
I've tried on a common Outlook account and it fails too, the received mail hasn't the headers sent.