Mail server settings are domain-specific. For each domain, you may specify a specific set of mail servers that hold the E-mail accounts and should receive E-mails for all accounts at that domain. If you plan on using a non-default mail option, you will have to apply changes to all domains that will not use the default option.
The mail server settings are stored as name server entries in the DNS system. Therefore, any changes to the mail server settings may take up to 48 hours to propagate worldwide.
Default Mail Server
This is the default option for all domains added to your package. Using this option, all E-mail are set to route mail to your dedicated server instead of the 1&1 Mail Servers. In order for your E-mail accounts to receive E-mails, you will need to install a mail service on your server, creating the accounts on your server and making sure that the service is running and is not blocked by any firewall. Unfortunately, 1&1 does not offer support for administrative tasks such as mail service setup and configuration.
1&1 Mail Server
You are able to change the mail settings to the 1&1 Mail Servers if you wish to manage your 1&1 Mail Basic account
through the 1&1 Control Panel instead of administrating a mail service on your 1&1 Server.
Other Mail Server
In the case where you already have a separate mail service that you use, you can select this option. Selecting this option will allow you to enter the mail server addresses that should handle E-mail transmission for the domain.
For additional information, you may want to reference: