Sending from Roundcube to my email at another server results in a bounce message, however the email is delivered.

======== Required information ====
- iRedMail version (0.9.5-1 ):
- Linux/BSD distribution name and version: Centos 6.2
- Store mail accounts in which backend (MySQL):
- Web server (Apache):
- Manage mail accounts with iRedAdmin-Pro?No -free version
- Related log if you're reporting an issue: /var/log/maillog
====

the bounce message:
This report relates to a message you sent with the following header fields:

  Message-id: <75493e88469cf0f996285979be37bd64@davomail.com>
  Date: Thu, 24 Nov 2016 23:20:40 -0600
  From: david@mydomain
  To: myusername@thierdomain.com
  Subject: roundcube to apple

Your message cannot be delivered to the following recipients:

  Recipient address: david@mydomain.com
  Original address:myusername@thierdomain.com
  Reason: Remote SMTP server has rejected address
  Diagnostic code: smtp;554 5.7.1 <david@mydomain.com>: Recipient address rejected: Policy rejection not logged in
  Remote system: dns;mail.mydomainl.com (TCP|17.142.194.250|52187|162.243.70.109|25) (mail.mydomaincom ESMTP Postfix)

Diagnostic-code: smtp;554 5.7.1 <david@mydomain.com>: Recipient address
rejected: Policy rejection not logged in

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I tried changing $config['default_host'] = 'localhost'; in Roundcube config to $config['default_host'] = 'tls://127.0.0.1';
but no luck