Because of the lack of Mailgun inclusion in the SPF record of the domain.
 Here’s how to check this –

  1. Look up TXT records of the domain using any DNS lookup tool: https://dnslookup.online/txt.html

  2. The record containing v=spf1 must have include:mailgun.org added

If this is not in place, there are a couple of solutions to fix this –

  • 1. Add include:mailgun.org to the SPF record on the main domain

  • 2. Set up a custom email domain for the referral program. Read more about it here.