==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail     1.3.2
iRedAdmin-Pro     4.7 (MySQL)
- Deployed with the downloadable installer?
- Linux/BSD distribution Ubuntu - 20.04
- Store mail accounts in which backend (/MySQL/L):
- Web server (Nginx):
- Manage mail accounts with iRedAdmin-Pro?  YES

====

I have a domain on my iRedMail server ( Example.com) and a user bob@example.com
I also have a domain bigOrganisation.com and website@bigOrganisation.com

I have set up MailGun to deliver emails on behalf of bigOrganisation.com website. (IRedMail kept throttling senders even with it turned off)

When the website ( bigOrganisation.com ) sends an email to bob@example.com, via mail gun, the MX record points to my mail server.  The email is sent as
FROM: website@bigOrganisation.com
TO: bob@example.com

My mail server rejects the email  with Sender address rejected: User unknown

Now if wordpress@bigOrganisation.com was an external mail sender, with no account on my server, it would get through okay.

How do we get this email accepted?

Thanks