Please note, this feature is only available in the Enhanced Accounts.
After logging in to your webmail, click the E-Mail link at the top.
Click on the three dots just above where the dates are on your received email. It looks like this:
Choose "Accounts" and you will see this:
On the first tab you will see your user information, then you type in the Name you want displayed for your additional incoming email, the email address, and you can create a text signature, if desired. You can set some additional settings before you click the Server tab.
This information is given to you by your other email provider so that you can check email from their server.
Click on Apply and then Close.
You now have setup an external account and it shall appear on the folder tree on the left, including separate Sent Items folder for it. When composing a new e-mail, you can select to use this secondary account instead of your primary one.
Note: If you only want to use a different sender when sending e-mails, without setting such as sender external account, the procedure is similar to that previously seen.
Click on that same three dots, then Accounts. Double click on your account and you will see a link a the bottom left called "Aliases"
Click Aliases, Click New and then this screen will show up:
Enter the Name, Email address and signature (if desired) and click Save. It will show up in your Aliases list now. When you compose an email, you can now choose the alias from the drop down list in the "From" area.