As you know the Mail app that is included with Windows 8.1 does not support POP3 accounts.
The following is the workaround by using Outlook.com.
With Outlook.com, you can link an existing POP account to your Microsoft Account.
Here are the steps:
1. Log into Outlook.com with your Microsoft Account
2. At the top right, click on the setting icon
3. Select “More Mail Settings”. This will bring up the inbox option
4. Select “Sending/Receiving mail from other accounts”
5. You will then see a page where you can add the details of the account. Make sure you click on “Advanced options”. This will provide you the field boxes for the POP configuration.
6. Configure your Windows 8.1 with the same Microsoft account that you used to log into Outlook.com (if it has not been done yet). 7. You should also consider changing the “Reply-to address” if you want to use the same address from the POP account. By default, Outlook.com will use the Microsoft Account address. This configuration can also be set on the “More Mail Settings” page.
Known issue regarding this solution
When replying to a message delivered to the POP3 address, the message will be sent from the Outlook.com account, not the POP account and may include a “on behalf of header” Example: From ABC@123.com on behalf of MY.POP3@account.com
The Mail app is only able to send from the account that it has the settings for. So when I reply to a POP3 message with my Outlook.com account via the Mail app, the messages is sent from the Outlook.com account and not the POP3 account and there is no on behalf of header. While there is an option to change the account in which you are sending from, there is not an option to select your POP account from the Mail app.