My contacts are receiving spam from me that I didn't send
While no one else can sign up for the same email address that you use, there are ways people can gain access to your account or send emails that look like they're coming from you. Be aware of how this happens and the steps you can take to stop it.
Someone accessed your account
If your Yahoo account is compromised, someone has either learned your password or has hacked into your account to send messages from your email address.
Someone created fake emails from you
Someone has the email addresses of your contacts and is spoofing messages that look like they're coming from you. They may have current or previous access to your account or have compiled the addresses from an email you've sent in the past.