If you want to use a 1&1 domain with mailboxes from a different provider, you simply have to enter the corresponding mail servers in the domain's DNS settings.
Using Different Mail Servers
To use different mail servers, you have to enter the provider's mail servers in the domain's MX record. This record determines which mail servers are responsible for receiving e-mails for this domain. Consult with your e-mail provider to find out which mail servers you need to enter for your domain.
1&1 Backup Mail Servers
To ensure that you can continue to send and receive e-mails should a mail server fail, you can create up to four MX records. If the e-mail provider does not provide four mail servers, you can also use the 1&1 mail servers as backup mail servers. These are automatically entered as mail servers 3 and 4 in the DNS settings.
In this case, the 1&1 backup mail servers save your e-mail traffic for 72 hours. During this time, our servers regularly attempt to return the e-mails to the mail servers that you specified as MX 1 and MX 2. If this is not successful, the e-mails are discarded after this time and are not delivered.
Entering MX Records
Select a domain from the My Domains page and then click the DNS tab.
Click Add Record.
Select the MX item from the Type list and enter the Host Name, Value,and Priority into the text boxes.
Select a TTL from the drop-down box and then click Save.
Your changes take effect immediately at 1&1 and should be updated everywhere within an hour.