Spam filter blocks outgoing mail

[Lon More]

For the past couple of days I get this error when trying to use my main mail account from my PC

An unknown error has occurred.

Subject 'Fw: update your account information'
Server Error: 550
Server Response: 550 Message refused by spam filter.
Server: 'mail.home.nl'
Windows Live Mail Error ID: 0x800CCC69
Protocol: SMTP
Port: 25
Secure(SSL): No

I guess this is a pfishing attempt and that someone is trying to use my PC to spread it. I can't find any trace of this mail anywhere. I'm using Avast firewall and anti-spam but there seems no way to reconfigure it.
Any help welcomed

[mixture]

Subject 'Fw: update your account information'

What were you forwarding ? Was it a piece of spam by any chance ?

[OldLurker]

Lon More, from what you say it sounds as if your PC may have acquired a 'spambot' (a type of virus) which is trying to send phishing messages, which are being rejected by your provider's mail server. Get your PC looked at by a professional.

[Capetonian]

I have sometimes had a similar message when trying to send out multi-addressed emails (typically >25) such as a newsletter. The solution is to split the recipients.

Whether this helps in your case, I don't know.

[Lon More]

your PC may have acquired a 'spambot'

That's what I thought, especially as i just dump that sort of message in the spam folder and would never forward it. However, if I can find where it originated the owner will start to get loads of offers from a throw-away address.
I've cleaned out all my folders and re-installed Mail however I still get the error. Looks like it's going to the local Geek for some fettling.

Thanks for all the help