To get the server name, you need to either hunt around in your documentation or online, or have a conversation. With whom? With either your Internet Service Provider (for your home or small business email account), or, for a work or school account, the Network Administrator of that system.

Remember, you want to confirm a) that the account uses a POP server -- and get the name of the server -- and b) that it does not require a secure connection (SSL).

Here's how the conversation should go. Roughly.

Note: It's not possible to use the external mail feature to check messages in another Yahoo! Mail account that has been set up for POP access.