Chances are that you need to set up a Sender Policy Framework (SPF). 

Your developers can set that up for your following this guide.

Using DMARC?

Larger domains typically will be using a DMARC record to add an extra layer of protection. If this is indeed the case, DMARC validation will fail due to misalignment of your 'from' domain and the d= domain. 

In this case, a possible solution is to send emails from a subdomain (e.g. @affiliates.your-domain.com) create the appropriate SPF record and then create a more relaxed DMARC record, specifically for that subdomain.

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