If your hosting plan includes Premium Email, your email server is different than indicated in your hosting documents. Premium Email offers connectivity via POP3, IMAP, and Premium WebMail.
POP3 Server: pop.1and1.com
IMAP Server: imap.1and1.com
SMTP Server: smtp.1and1.com
Your login name is your complete email address.
SMTP Authentication IS required.
How to Enable HTML Mail in Premium WebMail
How to setup a Signature in Premium WebMail
Outlook 2010 Setup Information
Thunderbird Setup Instructions (IMAP)
Each mailbox is 2GB in size. The maximum message size supported is 100mb.