One Yahoo account refuses to connect to mail app

I have two Yahoo accounts and an Exchange account.

One Yahoo account works fine, as does the Exchange account. They connect flawlessly to my iPhone 4S iOS7 device and push my mail to me.

However, my second Yahoo account refuses to connect. I have recently changed passwords, which sparked this, I believe. Although before changing passwords, after charging my phone, I would get notifications for emails to this account without being able to read the emails until I restarted my phone.

Anyway, I have tried entering my account in many ways:

This method produces the infamous "Server Unavailable; Please try again later" error message.

If I enter the wrong password though, it tells me so.

Then I added Yahoo's imap and smtp servers (imap.mail.yahoo.com/smtp.mail.yahoo.com)

And the relevant account info.

I tried making the "Username" field both my email with AND without the @yahoo.com extension.

This method produces the "Cannot Get Mail; The username or password for "imap.mail.yahoo.com" is incorrect"

I have quadruple checked my email (I have had this email for going on 9 years, I know it by now)

I have entered, re-entered, and re-re-entered my password. Carefully, precisely. I know it is correct.

This is a variation of the above method, except I used different imap and smtp servers.

I tried imap.apple.mail.yahoo.com/apple.imap.mail.yahoo.com/plus.smtp.mail.yahoo.com

All with the same info otherwise described above. And got the same password error.

I have restarted my phone many times. I have downloaded the Yahoo Mail app, and logged onto the account flawlessly. I have logged onto Yahoo.com and logged onto the account flawlessly. I have tried changing the password a SECOND time, just to check if the password was somehow conflicting with the iPhone mail app. I have tried this on 3G and two different Wi-Fi networks. All produce the same errors above.

If this has happened to anyone before, I'd love to hear about it, especially if there are fixes available.

Thanks!