How to Migrate Contacts from 1&1 Webmail to Office 365?

The steps as follows will tell you how to migrate contacts from 1&1 webmail to Office 365.

Log in to the 1&1 WebMail interface
If you have assigned your 1&1 Mail Basic or 1&1 Mail Business mailboxes to another domain or subdomain as described in the Migrating 1&1 Mail accounts to Office 365 article, you will have to use the correspondingly-modified e-mail address for logging into 1&1 Webmail.
Export your contacts from 1&1 Webmail in CSV format
You need log in to your 1&1 WebMail account to export the contacts of yours.
Open the exported CSV file with Microsoft WordPad and save it again immediately
This step is only necessary for importing the file in Outlook later.
Importing the CSV file in Outlook
Please refer Microsoft Office Help: Importing Contacts into Outlook

That's all!