If you received an error message (553) while sending an email via your external mail client, your alternate non-Yahoo! email address is not verified. To properly use your POP service, please follow these instructions.

1. Click Options in the upper-right corner of the page. If you are using the All New Yahoo! Mail, click "Mail Options" in the menu that appears.
2. In Yahoo! Mail Classic under “Management”, click Mail Addresses. If you are using the All New Yahoo! Mail, click "Accounts" and then "Add or edit an account".
3. Click Add.
4. In the box, enter an account name for your own reference (for example, Work, School, The One Where My Name Is Lenny, etc. – no one else will see this) and click Continue.
5. Look now: two boxes! In the first one, put the Name you’d like others to see when they get email from this address. In the second one (this is the part you’ve been waiting for), type the Email address you’d like send from – from within Yahoo! Mail! (You’ll be asked to verify this other address in an email we will send to you there.) Click Continue.
6. On the page that opens, check to make sure you’ve entered the correct email address. If you need to make any changes, click Edit Address. If it looks fine, click Send Verification to generate the email we send to you to make sure this is an actual account you use. We gotta do this to help prevent spam!
7. The “Verify Account” page opens. Here’s the easiest way to complete your setup and start sending mail from the new address:
1. Leave this "Verify Account" page open.
2. In a different browser window or application, sign into your other Mail account.
3. Look for an email from Yahoo! Member Services, and open it.
4. Copy the “confirmation code” in the last line of the email by either double-clicking it or clicking and dragging over it, then either clicking Copy from the Edit menu or pressing Ctrl + “c”. (Mac users press Command + “c”.)
5. Get back on the “Verify Account” page and Paste that confirmation code in the box beside “Confirmation Code:”. You can paste by clicking Paste in the Edit menu, or by pressing Ctrl + “v”. (Mac users press Command + “v”.)
6. At the bottom of the page, click Verify Account.
7. OK, last step! A sign-in page opens with your ID already filled in. Sign in with your password.
8. Hot, you’re done. See the new account in the list on your Mail Accounts page? Now you can easily send from it by opening a Compose page, clicking the down-arrow on the end of the “From:” box, and clicking the new account in the pull-down list of email addresses. That’s it! People receiving the message will see it as being sent from that address.