If you use 1&1 as your web hosting provider and you then sign up for Cloudflare (where you then update your authoritative nameservers to point to us), you may get an e-mail from 1&1 stating they will remove your account. To prevent 1&1 from sending you this email, 1&1 has created this article about using nameservers from another provider while hosting with 1and1.
This e-mail will look similar to the one below:
We at 1&1 Internet have noticed that you have changed the name
server of your example.com domain. Your new settings are:
DNS3: not set
DNS4: not set
Because the DNS settings with your Domain registrar and within 1&1
as your hosting provider are different, your website hosted with
1&1 Internet can no longer be reached via the example.com domain.
The e-mail addresses included in your package, if any, have
also been disabled.
If you still intend to keep using our services, you can enter the
following name servers with your original Domain registrar by
January 20, 2013 at 02:09:03 AM and continue using our services as usual.
Therefore just replace the above mentioned DNS settings with your
Domain registrar with the following 1&1 name servers:
If by January 20, 2013 at 02:09:03 AM you have not registered with our name
servers, we will remove the example.com domain from our
systems. If you want to use your domain with your 1&1 Package after
January 20, 2013 at 02:09:03 AM, you can specify this configuration on the
Control Panel once more and enter the name servers specified there
with your registrar.
If you want to keep the current name server settings, please make
sure that you are using the same Name Servers at the 1&1 Administration
as you use with your original Domain registrar."