*) Your iRedMail mail server hosts both domains: inbound.hong2.com, hong2.com.
*) If email has Sender address ends with '@inbound.hong2.com', or '@hong2.com', Postfix (MTA on iRedMail) asks the client (MailGun server, in your case) to perform SMTP auth to identity itself. Obviously, MailGun won't perform SMTP AUTH for this forwarded email.
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Re: Can't Figure Out Setting up Domain Alias for MySQL
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