Yahoo! s implementation of IMAP is a pretty weird one. IMAP is the ability to synchronize all your personal folders (not just the inbox) on an external client like Outlook, Thunderbird or Apple Mail. At first Apple announced that Yahoo! would offer free IMAP to anyone on the iPhone. Then they released the protocole on BlackBerry and Windows Phone. Recently it was discovered by some genius in the Yahoo! Mail group that IMAP was actually wide opened to anyone should you have a paid or a free account, should you set it up on a mobile device or on your computer, should you be using a 3G or a WiFi connection.
Now before you ask here is how to set it up:
Incoming mail : imap.mail.yahoo.com / SSL
Sending email : smtp.mail.yahoo.com / SSL
Make sure you’re using port 993
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UPDATE : As explain here, when setting up IMAP you’ll see a new folder called Y!Chats which aggregates all your messenger conversations for a more unified communication experience. We believe that this is gonna be implemented as a Yahoo! Mail application in the future
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Now, one problem lies in the push system which means that you receive near-to-live updates as soon a new email message has arrived in your mailbox. The push notification works fine when you set up Yahoo! Mail BUT it only takes into account new email messages that have arrived in the inbox.
In other word, if you have created filters in the Yahoo! Mail options to automatically redirect messages to certain folders, you won’t get any notifications.
So far the only way that I have find to go around this problem is to desactivate all those filters in Yahoo! Mail so that all messages actually reach the inbox. This way I get push notification on my iPhone. However I also have set up my Yahoo account in Mail.app on my Mac and created those rules directly within the software. Now once a week, all I have to do is to actually run Mail.app let it sync my email, select all of them and choose to apply those rules.
Some of my messages will be then be moved to my Yahoo! Mail folders and those modifications will be synchronized online. I like to get my mailbox clean and hopefully Yahoo! will do something about that. By opposition, when setting up a Google account on your iPhone you can actually choose which labels should be directly sync’ed to receive push notifications on your iPhone.