Finally got the response from 1and1 technical support. You have to setup a proxy server to make it work:

Use the following in web.config to establish a connection to the proxy server:

<system.net>
<defaultProxy>
<proxy
usesystemdefault = "false"
bypassonlocal="false"
proxyaddress="http://ntproxy.1and1.com:3128"
/>
</defaultProxy>
</system.net>