Double-click the Yahoo! Messenger icon on your desktop or in your icon dock.

To sign in using a different Yahoo!7 ID: Select Messenger Select Sign Out

Here's how you can allow someone else to sign in to Yahoo!7 Messenger on your computer: Sign out of Yahoo!7 Messenger by clicking the Mess...

If you are signed in to Yahoo!7 Messenger and open your Yahoo!7 Mail account on the same computer, Yahoo!7 Mail will detect thatMessenger 11.5 | Messenger 10 and earlier | Messenger for Mac

If you try all of the above and Messenger still can't connect, please contact your network administrator, ISP (Internet Service Provider), or whoever initially set up your computer to make sure that your firewall and/or proxy settings are correct.Messenger for Mac

This article explains what to do when you can't sign in to Yahoo!7 Messenger. To re-authenticate your Yahoo!7 Messenger :

If you'd like to chat in Yahoo!7 Messenger using one of your account's aliases instead of your main Yahoo!7 ID, enter your alias into the Yahoo!7 ID field when signing intoMessenger 11.5 | Messenger 10 and earlier | Messenger for Mac | Messenger Mobile | Messenger iPhone | Messenger Android

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To allow someone else to sign in to Yahoo! Messenger for Mac you'll need to remove your sign-in information and then have the new person enter their sign-in information: Click Change User/ LoginMessenger for Mac