You can’t, the BCC would be the only way with Yahoo! Mail.

CHOOSE BCC

BCC AND CC

Cc: stands for “carbon copy.” Anyone listed in the Cc: field of a message receives a copy of that message when you send it. All other recipients of that message can see that the person you designated as a Cc: recipient has received a copy of the message.

Bcc: stands for “blind carbon copy.” This is similar to the Cc: feature, except that Bcc: recipients are invisible to all the other recipients of the message (including other Bcc: recipients). For example, if you send a message To: and Bcc:, then johndoe sees himself as the message’s only recipient. Janedoe, on the other hand, is “in the know”—she can see that you sent the message To: johndoe, and that you blind-copied her. To add an entry in the Bcc: field, click the “Show BCC” link to the right of the “To:” field.

Note: To send a message, you must always specify at least one recipient in the “To:” field. If you don’t, an error message appears when you attempt to send the message.

choose compose
then you will see these option”s above TO when composing email BCC AND CC
put in the “to:” box the adresses.
make sure that each email address is separated by a comma.

MORE INFO

How about if the TO line says—Undisclosed-Recipient ???
You can do that this way —–go ahead and enter your own address on the TO line—-BUT—enclose it within parenthesis—like this—-
(yourname@anywhere.com)
Enter ALL other addresses in BCC.
The mail will be delivered showing Undisclosed-Recipients on the TO line.

These instructions are for Yahoo mail Classic:
and I have included the tutorials – so you can see
if needed- what I AM referring to:

You can create Categories in your Addressbook , then when you go to compose an email you just check the “categories”that you want to send to.

Getting started:
You put all of the email addresses in a group and one email can be sent-
Pasted from

go to
1. Open your Yahoo! Address Book.
2. Click Edit next to Categories on the left side of the page
(classlist)

3. Click the Add Category button.
The Add Category page appears.
4. Enter a name for the category, and select the order used to display the list of contacts in the new category (by first name, last name, or email address).
The new category appears on your list of categories.
5. When you finish adding categories, click the Done button to return to your Yahoo! Address Book.
NOW THE EMAIL AND HOW TO SEND THE EMAIL:
1. Go to Mail Open Compose Page
2. TO: [category] Put a check on CLASSLIST (CATEGORY CREATED)
3. COMPOSE EMAIL
4. SEND

EACH PERSON ON THAT LIST SHOULD RECEIVE AN EMAIL!
***An Idea would be to tell them to add YOU to their address list so that it doesnt go to spam, and if they claim that you they did not get the email tell them to check SPAM box for it!

these 2 tutorials – are the best ones to review so you understand! i have just been doing it for a while

WORKING with Categories
Composing the email

Check out a program called PC iMail, which will send each message individually to each address, and you can personalize your message to include each person’s name and address using mail merge.

It will also let you configure a time delay on your message delivery. That way it’ll send say 5 messages every minute (or however many you want), so that you’ll be in compliance with your ISP’s limitations for mass mailings.

The trial version gives you 30 runs, and if you just leave the program running then it’ll last a LONG time