How do I stop a spammer from using my email to send out mass spam?

Catherine: How do I stop a spammer from using my email to send out mass spam?
I signed on this morning and had lots of undeliverable returned mail, which I didn’t send out. I went to my send box and there were lots of emails sent out as me (though I wasn’t online at the time). I understand this means someone is using my email address as the reply to, but how can I have this stopped?

Answers and Views:

Answer by ssj7
Change your Password , Also UnCheck the “Sign me in Automatically” or “Remember Me”

Answer by lucee
Aw, come on – changing your password won’t lock them out – notifying Yahoo might.
MAIL PROBLEMS
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MAIL BASIC HELP
https://help.yahoo.com/kb/index?page=product&locale=en_US&y=PROD_MAIL_ML&pir=ZvVJIk9ibUm_nhoCQdeuISoVozO2KzHKLZK6xQ–.
MAIL ABUSE
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Answer by Lyn G
Sounds like your E-mail account has been compromised, or you picked up a virus, Catherine.
Immediately have your computer checked for a ‘bug’ or ‘bugs.’ If you have the proper programs, do the scans yourself, if not you need to take your computer to some one who can.
Once it is clean of pests, install a firewall and an anti-virus program if you do not have one yet.
I strongly recommend a program for malware and adware also.
After you finish this, then go in and change your password, using a good strong one.
Also, check your account information and make sure no information was changed, if it was change it back immediately.
Then, you will have to inform your contacts that it was not you sending that mail, but they need to check their computers in case they were also infected.