The original message from Yahoo needs a bit of clarification Here’s the original:
http://help.yahoo.com/l/us/yahoo/mail/original/manage/sendfrom-07.html

 

If you received an error message (553) while sending an email via Outlook, Thunderbird or other external email client, your alternate or Yahoo! Mail address is not verified. To properly use the Yahoo! SMTP service, please follow these instructions.

  1. Access Yahoo! Mail by logging in (http://mail.yahoo.com) with your ID and password.
  2. Once you're logged in, click Options in the upper-right area of the page.
  3. 1
    1. If you are using new Yahoo! Mail, click Mail Options, then Accounts from the left-side menu and then the Add or edit an account link in the middle of the page.
    2. 2
    3. 3
    4. If you are using Yahoo! Mail Classic, click Mail Addresses under "Management".
  4. Click Add.
  5. In the Account Name field, enter a name for your own reference (for example, Work, School, My Domain, etc.) and click Continue. 4
  6. In the Name field, enter a name for outgoing mail from this address (this is the name the person receiving email will see).
  7. In the Email field, type the alternate email address you would like send mail from THE ONE PEOPLE SEND THINGS TO YOU IN OUTLOOK (we'll send a confirmation code to this email address, which you'll use to complete this setup). Click Continue. 5
  8. Click Skip this step:
  9. 6
  10. On the next page, click the Send Verification button. This transmits a verification email from Yahoo! Member Services to the email address you just specified. Here's the easiest way to complete your setup and start sending mail from the new address: 7
    1. Leave the "Verify Account" page open.
    2. Retrieve the email sent by Yahoo! Member Services to your alternate email address. 8
    3. Enter the "confirmation code" from this email into the Confirmation Code field in the "Verify Account" page in Yahoo! Mail. 9
    4. Click Verify Account.
    5. Sign in with your Yahoo! Password on the final page.
    6. 10
    7. Your alternate email address is now verified and can be used for sending emails through Yahoo! Mail or your email software. 11

Note: Every primary and secondary Yahoo! Mail ID can verify up to 10 alternate email addresses each.
Special notice for Outlook 2003 users: You might need to download a program update to resolve this problem. Please click here for detailed instructions.