To sign into Yahoo! Mail for SMS, compose an SMS message on your mobile phone with these contents:
in Yahoo! ID password
For example, if your Yahoo! ID is chetan and your password is mypass379, your screen would look like this:
in chetan mypass379
Send the message to 8243. You'll receive a confirmation message on SMS that your sign-in was correct, or an error message if it was incorrect.
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Once you have successfully signed in, send the command mail to 8243 to get the list of messages you have received. Now you can select a message to read from the list. You will receive the first 160 characters of the message.
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If you want to send a message to a Yahoo! ID, the "@yahoo.com" is optional. For instance, to send a message to your friend whose Yahoo! ID is niraj saying "I'll see you later", your screen would look like this:
to niraj i'll see you later
Send the message to 8243.

You can find more about it here...
in.mobile.yahoo.com