======== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.7
- Linux/BSD distribution name and version: Debian 7
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? Yes
====

Hi all,

we have a strange behaviour from our iredmail server.

Our server hosts domains:

a.com
b.com

user@a.com send an e-mail to receiver@c.com.

All e-mails from reveiver@c.com are forwarded to receiver@b.com.

The external Mailserver at c.com receives the e-mail and tries to forward it to receiver@b.com.

This fails with:

This is the mail system at host c.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system
<receiver@b.com>: host mx.srhosting.eu[xx.xx.xx.xx] said: 554
    5.7.1 <receiver@b.com>: Recipient address rejected: SMTP AUTH
    is required, or it is a spam with forged sender domain (in reply to RCPT TO
    command)

Now, the way i see it is that the e-mail is being denied, because c.com is not in the list of authorized senders for domain a.com. But it's a simple forward ?!?

Any ideas ?
thanks.