>... is there a way for me to configure yahoo email so that outgoing
>mails cannot have multiple recipients?
Interesting suggestion. But would that be a permanent and unalterable setting?
>This would probably stop this spam going out, wouldn't it?
One possibility is that the spam is "spoofed" -- meaning that it didn't actually originate in your Yahoo Mail account, even though it appears to have. In that case no change to your account would have any effect.
If your account was actually used, then the person or program that accessed your account could turn off the setting. Unless the setting is permanent, or requires some additional security to change it.
Too, if the spam is being generated by a program, it could be designed to send each outgoing message separately, to just one recipient at a time. Programs are really good at tedious tasks like that.
>It has happened to me three times this year so far, I'm at my wits end.
>No amount of anti virus and spyware software seems to protect me, what
>am I doing wrong??
If the spam is spoofed then there's nothing you are doing wrong or could do to stop it. It can and does happen to anyone.
If your account was actually used there are a couple of possibilities.
One of them is "session hijacking". This can happen if you are signed in and someone gains the ability to "eavesdrop" on your session. Possible, but not very likely on a wired connection, but increasingly a problem on Wi-Fi connections, particularly public "hot spots" that are not locked down. Note that this form of attack doesn't involve putting any kind of virus or spyware on your computer. To protect against this you need a secure connection end-to-end, either using HTTPS: protocol or a VPN tunnel to a trusted server.
And of course, there's always the possibility that your computer is infected with something that has evaded the anti-virus and/or anti-spyware tools you've tried. It could even be a hardware device connected to your computer, if your computer is ever in an unguarded location, or you ever use library, cafe or kiosk computers.