From the Opera Menu, select Mail and Chat Accounts.
Opera start page
If this is your first time adding a mail account to Opera, you are prompted to create a new account. Click on Yes.
If you already have a mail account configured in Opera, the Manage Accounts window will open and click on Add.
Account Needed prompt
Select Email as the type of account you would like to add and click the Next.
New Account Wizard
Enter your name in the Real name text box. This is the name that others will see when receiving your mail.
Enter your full 1&1 Mail Basic
address in the Email address text box.
Enter an Organisation name if applicable. Otherwise leave the text box blank.
Click on Next to continue.
Enter your full 1&1 Mail Basic address in the Login name text box.
Enter your 1&1 Mail Basic account's password in the Password text box.
Select the POP radio button.
Click on Next to proceed.
Enter pop.1and1.co.uk in the Incoming server text box and check the box for Use secure connection (TLS).
Enter auth.smtp.1and1.co.uk in the Outgoing server text box and heck the box for Use secure connection (TLS).
Click on Finish to complete the E-mail account setup.
The Mail sidebar in the Opera window and your newly-added 1&1 Mail Basic account should be listed in the sidebar. Click the account to view the E-mail folders such as the Inbox.
For additional information, you may want to reference: