On your computer, click the Start Menu and select Control Panel.
If using Windows XP, double-click the Mail icon.
If using Windows Vista, Windows 7 or above, type Mail into the search box at the top-right of the Control Panel window to quickly find the Mail icon. Then, double-click it.
Click the E-mail Accounts button.
Select your 1&1 e-mail account that you have already set up and click Change.
Click the More Settings button.
In the new window that opens, click the Outgoing Server tab at the top.
Check the box for My outgoing server (SMTP) requires authentication and ensure that Use same settings as my incoming mail server is selected. Click the Advanced tab to configure addtional settings.
Enter port 995 as the Incoming server (IMAP) port and check the box for This server requires an encrypted connection (SSL).
Enter port 587 as the Outgoing server (SMTP) port and select TLS from the Use the following type of encrypted connection drop-down box.
Click the OK button to save your changes.