Hi, from time to time I receive following error while I try to send an email (normally using Thunderbird or Outlook - not bulk mail) to some Gmail mailbox: subject: Undelivered Mail Returned to Sender (my actual hostname was replaced with similar casrv02.myserver.com) I think it is related to the improper configuration of IPv6. The obvious solution is to disable IPv6 or to do some additional configuration. The problem might also be that my reverse DNS is casrv02.myserver.com while the domain I send out the email from is firstname.lastname@example.org (different from reverse DNS). Can you please help me out how to disable IPv6 or adjust the configuration for ISPConfig? (if that is the issue in your opinion) My setup: Debian 7 x64, OVH VPS Cloud Installed following these howto instructions: The Perfect Server - Debian Wheezy (Apache2, BIND, Dovecot, ISPConfig 3), nothing customized.