Specifically, when you click Add Other Account..., a sheet appears with options for adding accounts related to mail, messages, calendars, and contacts. (You'll also see an option to add an OS X server account--a topic we'll assiduously avoid for the time being.)
If you have an email account that you'd like to add, select Add a Mail account and click the Create button. Just as with a preconfigured service, a sheet then asks for your name, email address, and password. Fill these in and click Create.
If you're lucky, OS X will be aware of this service and will configure the service's settings for you. If fortune isn't in your favor, you'll see yet another dialog box informing you that Mail couldn't discover the account settings. In that case, click the Continue button.
Mail will launch and produce an Add Account window. Within this window you must choose the account type (POP, IMAP, Exchange, or Exchange IMAP) and enter the incoming mail server address (I'll discuss what these are shortly). Your username and password will be entered automatically. Click Continue, and Mail will make sure that the settings you've entered allow it to communicate with the service.
Click Continue once more. You'll be asked to choose the kind of incoming-mail security scheme the service employs (password authentication, for example). Click Continue again, and you're asked for the service's outgoing-mail server address. Enter it and click Continue, and Mail will make sure that it can communicate with the outgoing-mail server. Once you see the confirmation, click Continue to be prompted for outgoing-mail security settings. Yep, click Continue again when those settings are confirmed, and you'll see an Account Summary where you can eyeball all the settings you entered. Finally, select the Take Account Online option and click Create to create the account in Mail.
I apologize if I sound like your grandfather relating stories of how he tramped through waist-deep tar to attend school the next county over, but this is the way we used to do things: If you wanted to set up an email account, you had to know an Internet service provider's server addresses and security settings. And, in some cases, you still do.
Fortunately, such information isn't a big secret. ISPs post the details on their websites, usually in a support area. Good ones will tell you how to set up their service with specific email clients. (You'll probably be using Apple's Mail.) If you can't find this information, give the ISP a call (you like waiting on hold, right?), tell the representative you're using a Mac, and ask for this information:
Do I have a POP or IMAP account?
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