ConfTool Does not Accept any E-Mail Adresses in the User Registration Form

Users try to register with ConfTool, but (suddenly) no e-mail addresses are accepted.
How can we solve this problem?

In the standard configuration, ConfTool tests the domain name of each newly entered e-mail address (i.e., the part after the @ in the address, like @googlemail.com).
The domain name has to exist, otherwise the e-mail address is definitely wrong and cannot be delivered.

Sometimes there are problems with this method in local installations of ConfTool, as the DNS server does not work properly or PHP has problems to access it.

To solve the problem, please carry out the following steps:

• Test if the local DNS server works or if DNS requests are forwarded to a valid DNS server. You can use the commands "nslookup" or "host" on the command line to do so. Fix this problem, if possible.
• If you cannot make any changes here / provide a working DNS server for your PHP installation, please disable the DNS test in the configuration file conftool.conf.php.
  You find the corresponding file in the directory "conftool/etc". The command is called:
  $ctconf['mail/checkdns'] = true;
  Please set it to:
  $ctconf['mail/checkdns'] = false;
  Please also refer to: Installation Instructions