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Are you setting the correct ports and the correct, full email account?<br />
<br />
<a href="http://email.about.com/od/macosxmailtips/qt/et_free_yahoo.htm" rel="ugc">http://email.about.com/od/macosxmailtips/qt/et_free_yahoo.htm</a><br />
<a href="http://www.pop2imap.com/yahoo_thunderbird.php" rel="ugc">http://www.pop2imap.com/yahoo_thunderbird.php</a>
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Are you setting the correct ports and the correct, full email account?

http://email.about.com/od/macosxmailtips/qt/et_free_yahoo.htm [http://email.about.com/od/macosxmailtips/qt/et_free_yahoo.htm]
http://www.pop2imap.com/yahoo_thunderbird.php [http://www.pop2imap.com/yahoo_thunderbird.php]

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I can't say that's it but the same thing happened to me with a new Yahoo mail account .. iphone vs. Windows PC.
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I can't say that's it but the same thing happened to me with a new Yahoo mail account .. iphone vs. Windows PC.

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I'm trying to help resolve a Mac Mail issue:<br />
The user's Yahoo! account works on the web. &nbsp;Can sign into Yahoo! mail that way.<br />
However, on their MacBook Pro OSX 10.6... it behaves differently. &nbsp;Doesn't work. &nbsp;This occurred just very recently after it working for a long time.<br />
I removed all of the mail accounts on the computer and started over (I think) using the exact same username / email address and password (enough times to prove there were no typos) and the problem remains. &nbsp;<br />
The error message is something like: &quot;can't connect to the server&quot;<br />
<br />
How to fix? &nbsp;This is a new one for me!
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I'm trying to help resolve a Mac Mail issue:
The user's Yahoo! account works on the web.  Can sign into Yahoo! mail that way.
However, on their MacBook Pro OSX 10.6... it behaves differently.  Doesn't work.  This occurred just very recently after it working for a long time.
I removed all of the mail accounts on the computer and started over (I think) using the exact same username / email address and password (enough times to prove there were no typos) and the problem remains.  
The error message is something like: "can't connect to the server"

How to fix?  This is a new one for me!

Mac Book Pro - Mac Mail - Yahoo Mail

Mac OS X is the desktop operating system from Apple Inc., found on Macintosh computers. OS X consists of a Mach/BSD-based kernel, operating system interfaces primarily based on FreeBSD and additional frameworks (written in C, C++ and Objective-C providing user interface and application-level services.

Mac OS X

Apple's early computers ran on a variety of operating systems, including a Pascal version and one based on CP/M, before releasing its ProDOS with the advent of its 16-bit computer. With the release of the Lisa, which had its own OS, Apple and computing entered the graphical age; it was followed by the Macintosh, which offered a series of "System" operating systems. Other Mac operating systems included PowerPC until Apple acquired NeXT and folded it into the Mac OS. Current operating systems for Apple devices are Mac OS-X and iOS.

Apple OS

An email client, email reader or more formally mail user agent (MUA) is a computer program in the category of groupware environments used to access and manage a user's email. A web application that provides message management, composition, and reception functions may internally act as an email client; as a whole, it is commonly referred to as webmail. Likewise, email client may be referred to a piece of computer hardware or software whose primary or most visible role is to work as an email client. Email clients can also have other systems, like calendars, notes and contact managers.