Zhang, thank you - is there a way to make this work?
ZhangHuangbin wrote:
*) 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.
