Spamhaus.org blocking an IP

Are you on a fixed IP and run your own SMTP server for sending email?

Most people send email via an ISP or some sort of outfit which runs an SMTP server, and it is that IP which gets blocked if the server ends up transmitting a lot of spam.

If your own IP is a fixed one and has been blocked but you are not knowingly sending emails out from it, then your PC may have been taken over by a trojan which is running an SMTP server on it. There are millions of such infected PCs.

Check the SMTP server you are using the send out e-mails.

Are you actually sending emails out of your PC, using an SMTP server running on your PC? That would be highly unusual, and is not recommended due to dynamic IPs being routinely blacklisted / spam scored for antispam (because SMTP servers running on them are normally bots). And “fixed” IPs are often allocated out of dynamic IP blocks (by configuring the ISP’s server to dish out the same IP every time, to a dynamic IP customer) and thus get blacklisted / spam scored accordingly.

You should transmit emails via a third party SMTP AUTH service which ideally also supports DKIM signatures. For example authsmtp.com is good and is about 50 quid a year. The DKIM config is a bit of a painful process, because they need to validate you, but can be worth doing. It requires that you run your own domain and have access to the control panel for its DNS, but that is true for setting up SPF too, and correct SPF is a must these days.

Antivirus software fails to find many infections, because the viruses cloak themselves. The only somewhat reliable method is to boot the PC from a CD or flash stick containing a bootable virus scanner. Kaspersky do a free one which you can download, burn the ISO to a CD and boot the PC from it. But if your PC is properly infected, it is often not possible to disinfect it.