You can get blacklisted for spamming, even when you've not knowingly done so, in one of three ways:
1. Hackers can access your computer and send spam out from it (using your email address), without you even knowing. That's probably quite rare but you should run a full scan with your anti-virus software just to be sure that everything is OK. Also download, install and run the free version of Malwarebyte's Anti-Malware to check for malware on your computer:
http://www.malwarebytes.org/products/malwarebytes_free/
(NB: When installing that program, deselect the pre-ticked option to install a trial version of the 'PRO' version).
2. Spammers simply 'borrow' your email address (without hacking into your computer) to enter into the 'From' field of millions of spam emails. They do so either by getting hold of your email address from a website that you've signed up to or, probably more often, by simply guessing at what might be a valid email address.
[If either '1' or '2', above, applies, you'll need to get a new email address, so that you're no longer using a blacklisted one. GMX provides an excellent free email service. If you want an address ending in .co.uk sign up here:
http://www.gmx.co.uk/
If you'd prefer a .com address, go here:
http://www.gmx.com/ ]
3. Sometime spam blacklisting isn't based upon your email address but upon the dynamic IP address assigned to you by your ISP. (Your ISP only has a limited number of IP addresses available. Each user is assigned one of those addresses whenever they connect to the internet. You might currently have an IP address which was previously used by a spammer). Go here to see what your current IP address is:
http://whatismyipaddress.com/
Then disconnect your router from the phone line (or cable), wait a couple of minutes and reconnect. Then use the same link to confirm that your IP address has now changed.
Chris