Follow the steps below to enable SSL Encryption for Outlook 2007.
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 to find Mail. Then, double-click it.
Control Panel > Mail
Click on E-mail Accounts.
Mail Setup - Outlook
Select your 1&1 Mail Basic account that you have already set up and click on Change.
Overview of your E-mail accounts
Click on More Settings.
Internet E-mail Setting (IMAP)
In the new window, click on Outgoing Server tab at the top.
General Settings tab
Check the box for My outgoing server (SMTP) requires authentication and ensure that Use same settings as my incoming mail server is selected. Click on the Advanced tab to configure additional settings.
Outgoing Server tab
Enter port 993 as the Incoming server (IMAP) port and ensure that SSL is chosen from the Use the following type of encrypted connection drop-down box.
Choose TLS from the drop-down box next to Use the following type of encrypted connection and enter 587 as the Outgoing server (SMTP) port.
Click OK to save your settings.
Advanced Security Settings
For additional information, you may want to reference: