Hi there Kent and welcome
The problem is they technically did not send the emails from with your account.
They likely had access at once time, and once they had this they got a list of your contacts.
Then with this list they send out fake emails, they look like they come from you, but they do not. It is easy to do and you can even do it yourself using this link
you can see there that you can set any from address you wish and it will look like it came from that account. Of course they use sophisticated programs to do this on a massive scale.
Now that they have your contacts, its not possible due to this, to stop it ever happening again, you will just need to inform your contacts to ignore any suspicious emails.
For protecting your account, make sure you have a strong password ***** numbers and characters in it.
I would also advise you to enable Yahoo 2 step verification which makes it very hard for people to gain access to your account
You can do this by following my instructions here
this will avoid any future breaches of your account.
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.