Sending to all contacts

Presuming you're logging into webmail with your browser, the only way to add contacts to an email is manually, one at a time.  You could make multiple Lists, or groups but again, making those lists requires one entry at a time.

However, be aware that Yahoo will limit how many addresses you can send to on one email.  They don't publish that limit, either.  All you can do is try some number of contacts and see if it flies.  Also, if you send a bunch of these emails in a short period of time, again the details of which are not published, Yahoo may stop you from sending email for some period of time.

If you haven't figured it out by now, Yahoo does not want you to use their system to send mass mailing. 😁

Thank you!