Somebody Sent An Email Using My Account To All My Contacts. How Can I Stop This

We have determined that the links below may contain an answer to this question. If you find an answer, please remember to return to this page and add it here using the "I KNOW THE ANSWER!" button above. It's that easy to earn points!

• Why am I receiving an error 553 message when I send mail using an email program? | Yahoo! Mail Classic Help
  ... an error 553 message while sending email using an email program, either your Yahoo! Mail address or alternate email address (from a different Internet service) hasn't been...your own reference (for example, Work, School, Second Account), and click Continue Complete the following information:... Account page appears, and email containing the confirmation code is sent to the account you named. In...
• How can I stay in my Yahoo! Mail account and send email from another email address? | Yahoo! Mail Classic Help
  ...you to make sure this is an actual account you use. The "Verify Account" page opens. Here's...open it. Copy the "confirmation code" in the last line of the email by either double-clicking it or clicking and... addresses. People receiving the message will see it as being sent from that address. OK, so you also..." (or both), or contact that email provider directly. Next, type your user name for that account...
• Why am I receiving an error 553 message when I send mail using an email program? | Yahoo! Mail Help
  ... an error 553 message while sending email using an email program, either your Yahoo! Mail address or alternate email address (from a different Internet service) hasn't been...your own reference (for example, Work, School, Second Account), and click Continue Complete the following information:... Account page appears, and email containing the confirmation code is sent to the account you named. In...