Enter the name you wish to display to others when sending mail, your full e-mail address, and the password for the e-mail account. The description will mirror the e-mail address by default, although you may change it to whatever you like.
When finished, select Next at the top right.
The Next button is in the top right corner of the screen
Wait while the phone saves and then it should display a new screen with extra settings. Make sure that the IMAP button is selected at the top left and you should see the Name, Address, Description and Password should have carried over.
The IMAP button is to the left of the POP button
Scroll down and enter "imap.1and1.com" without the quotes in the Host Name field and your full e-mail address in the User Name field under the Incoming Mail Server section.
Enter "smtp.1and1.com" without the quotes in the Host Name field and your full e-mail address in the User Name field under the Outgoing Mail Server section and enter the password for the e-mail account.
The Outgoing Mail Server section is at the bottom of the screen
Then click Next at the top right.
If you receive a message that the iPhone Cannot Connect Using SSL click the Yes button to try without SSL.
The Yes button is to the left of the No button
Turn Mail syncing ON.
Optionally, you may also turn on Notes syncing.
When finished, click the Save button to continue.
The Save button is in the top right corner of the screen
The account will verify and you will be taken back to the main screen for Mail, Contacts, Calendars and see your newly set up account listed there.
The account is located below the iCloud button
Your 1&1 e-mail account is now connected to your iPhone and you can access it at anytime.
For additional information, you may want to reference: