It is possible to get direct Yahoo! IMAP access.
Yahoo! operates IMAP servers (imap.mail.yahoo.com in particular), which are globally accessible.

However they require a specific, but non-standard IMAP command to be sent before login is done. The command is: “ID (“GUID” “1″)”

// C#
imap.SendCommand(@"ID (""GUID"" ""1"")");

' VB
imap.SendCommand("ID (""GUID"" ""1"")")

You can also an IMAP over SSL on the standard port 993.

Here’s the full C# version of the code:

using (Imap imap = new Imap())
{
    imap.Connect("imap.mail.yahoo.com");
    imap.SendCommand(@"ID (""GUID"" ""1"")");

	imap.Login("user", "password");

	imap.SelectInbox();
	List<long> uidList = imap.Search(Expression.All());
	foreach (long uid in uidList)
	{
		IMail email = new MailBuilder()
    			.CreateFromEml(imap.GetMessageByUID(uid));
		Console.WriteLine(email.Subject);
	}
	imap.Close(true);
}

and VB.NET version:

Using imap As New Imap()
	imap.Connect("imap.mail.yahoo.com")
	imap.SendCommand("ID (""GUID"" ""1"")")

	imap.Login("user", "password")

	imap.SelectInbox()
	Dim uidList As List(Of Long) = imap.Search(Expression.All())
	For Each uid As Long In uidList
		Dim email As IMail = New MailBuilder() _
			.CreateFromEml(imap.GetMessageByUID(uid))
		Console.WriteLine(email.Subject)
	Next
	imap.Close(True)
End Using

Differences:
- non-standard command ID command is required before any other command.
- Examine explicitly requires CLOSE command (Imap.CloseCurrentFolder), otherwise subsequent SELECT (Imap.Select) has no effect and mailbox is still in read-only state
- IDLE is not implemented.
- SELECT for not existing folder does not create new folder.

You can download the latest version of Mail.dll .NET IMAP client here