Email - domain set as domain alias must not be offered for Email Forward creation
Some problem like in
http://bugtracker.ispconfig.org/index.php?do=details&task_id=2046 and http://bugtracker.ispconfig.org/index.php?do=details&task_id=1903 sorry i haven't reported
When domain2.tld is set as Domain Alias to domain1.tld then it should not be offered in list of source domains in Email Forward=>Add new Email forward