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Why am I getting failure to deliver notices when I did not send any email?
Posted: July 14th, 2016
You may have opened an infected mail or an attachment. Your address may have been found on someone else's infected computer. Some malware can install itself just by opening it, without even clicking a link! (It only takes a few hours to crack an 8-character password!) If the mail goes to your Contacts list or you receive Mailer Daemon bounce-back failure to deliver messages, you have had your account hacked temporarily - usually all they wanted was your address list. The mails will stop when they move on to spam other victims.
Deleting your account or Contacts List will NOT stop this spam - once the spammers have the addresses, they will continue to use them and yours for a while. Spam sent to your Contacts will also stop in a while, but it is annoying. Deleted addresses are in a list for your account still unless you delete them form the Deleted addresses file too.
Yahoo's advice is to change your password, but there are several more security settings to change than just your password. This Help page has more information, stated clearly, with several links to clarify the steps.
1) check that the alternate e-mail address/mobile phone number is still yours, under Options, Mail Options, Account Information, but then change it! (so the hacker will not be notified of the change). Check the 'Reply To:' address as well ...
Then change both your password AND your secret questions and answers. Make them unique and long, strong ones with mixed characters! Go to "Update password-reset info" to change your Secret Questions and Answers ...
*** Changing your password also deletes the cookie which apparently inserts that sly trojan address grabber ... ***
If you used this password for other accounts, change them too - make every one different. Make the password much longer than 8 characters, with mixed symbols ...
Usually, this is all you need to do, although mail will still be sent in the name of the old account for a short while
2) If spam mail was sent to any of your contacts, apologize, and notify all your contacts NOT to open short e-mails, especially those with no subject, and definitely not to click any links. Warn them to change their passwords and scan for malware too, just in case. Set up an alternate 'alias' account to use instead. This allows you to use a different address for sending, but still keep all saved mails and contacts and the first address active too for incoming mail. Your friends can trust this new address! There is a choice of address in an arrow in the From: box. Also add a signature to your mail so your friends will be sure the mail is really from you.
3) Just to be very sure your computer is not infected, (which is very unlikely, but a wise thing to do),run both an anti-virus and spyware/malware scans, updated and separately - even more than one malware scan, or Trend Micro's Housecall for an online a-v scan.
If you cannot download to your own machine, download and save on a clean computer, and transfer the executable file to a flash drive to use for your own ...
*** in 'Safe Mode with Networking' is the only truly effective method - click F8 repeatedly when the screen first lights on boot-up, then arrow keys and Enter to navigate. ***
Re-boot into Safe Mode for each separate scan. Instructions can be found here too ...
http://www.getwellsooncomputer.com/
These are the best free anti-malware programs for cleaning infections, and can be installed without conflict with other security programs. Click the link on the page for the FREE downloads. They works as well as the paid version, except that they do not provide real-time constantly-running protection as the paid version do.
This sourced from Yahoo Support Group: