When sending an email using yahoo mail program ,

please try first to send a test mail to your own email address.if you were able to receive the test mail, it's not the yahoo issue.you need to ask the receiver to check his/her list of blocked email addresses. it could be that yahoo was included in the list.

From Yahoo:

By entering the code you see in the box, you help us prevent automated programs from using Yahoo! services. This ensures better performance of Yahoo! services for you. source

Basically the reason for the code is to ensure it is a human sending the message, and not a virus/other type of malicious program that could use compromised email accounts to send massive amounts of spam.

You can use your email program that came with your windows program. It can automatically setup your yahoo account longest you know your user name and password.

Yahoo POP3 and SMTP Settings:

Find below the basic POP3 settings that you need to configure in the email program where you want to access Yahoo mails.

Incoming Mail (POP3) Server: pop3.mail.yahoo.com (Use SSL, port: 995)

Outgoing Mail (SMTP) Server: smtp.mail.yahoo.com (Use SSL, port: 465, use authentication)

Account Name/Login Name: Your Yahoo! Mail ID (your email address without the "@yahoo.com")

Email Address: Your Yahoo! Mail address (e.g., user@yahoo.com)

Password: Your Yahoo! Mail password
Note that you also need to enable "Web & POP Access" on your Yahoo Mail account to send and receive Yahoo! Mail messages through any other email program.

the error message means that the person or persons you are sending the mail to have problems with their mails so try sending it batch by batch or use a mass email sender software

It will be that the receipient hasnt opened the email maeesage if you have receipt of message facility active, or that the email address you have is wrong or typed in incorrectly. Either way, if the message hasnt reached the receipient's mail server then you wil get an error message saying this.

Incoming mail server settings

•POP server: plus.pop.mail.yahoo.com
•Use SSL
•Port: 995

Outgoing mail server (SMTP) settings

•SMTP server: plus.smtp.mail.yahoo.com
•Use SSL
•Port: 465
•Use authentication
•Account Name/Login Name: Your Yahoo! Mail ID (your email address without the "@yahoo.com", for example, “testing80”)
•Email Address: Your Yahoo! Mail address (for example, testing80@yahoo.com)
•Password: Your Yahoo! Mail password

Troubleshooting tips

If you still can’t send Yahoo! Mail from your mail program, you may be accessing the Internet through an ISP that is blocking the SMTP port. If this is the case, try setting the SMTP port number to 587 when sending email via Yahoo!'s SMTP server.

Your email program gives you the option of either deleting your Yahoo! messages from the Yahoo! server after downloading them into your mail reader, or leaving them on the Yahoo! server so you can see them online in your Yahoo! account, too.

AddressGuard is another powerful line of defense against spam:

• Create disposable email addresses in a snap
• Give them out when you don't want to give out your primary Yahoo! Mail address
• Have messages delivered to your Inbox or a personal folder
• Simply delete them if they start receiving spam

If the email receipiant has reported you as spam that is why you can't get through

you will have to call the receipiant and ask them to remove you from their spam list

If you use Yahoo! Mail for all your email needs, some of the amenities of Windows seem to elude you. A click on an email link in your browser brings up Outlook Express or another desktop email program, not a new message in Yahoo! Mail. Trying to send a file directly from Word, OpenOffice.org or another office application yields the same, unsatisfactory result.

Fortunately, the state of dissatisfaction need not be permanent. It will be overcome by making Yahoo! Mail act as your default email program in Windows, which, if you use Windows 95 or later is an easy thing to do.
Make Yahoo! Mail Your Default Windows Email Program

To set up Yahoo! Mail as your default email client in Windows:

* Download and install Yahoo! Messenger.
* Select Tools | Internet Options... from the menu in Internet Explorer.
* Go to the Programs tab.
* Make sure Yahoo! Mail is selected under E-mail.
* Click OK.

Yahoo! as Default Email: Does Not Work under Windows Vista

Unfortunately, Yahoo! Mail as your default email program may be hard-wired to use Internet Explorer instead of your default browser, and it does not work in Windows Vista.
Make Yahoo! Mail the Default Email Composer in Mozilla Firefox

In Mozilla Firefox, you can turn to the Mailto 2 Webmail script for Greasemonkey to have Yahoo! Mail handle email links (even under Windows Vista).

http://email.about.com/od/yahoomailtips/qt/et_default_mail.htm