Why have other people claimed I have sent them emails that I know I have not?
Your PC may have a virus or a 'trojan', or may have been used as an 'open relay server'. We advise you to use virus protection software to ensure you have no trojans on your PC.
Alternatively, your email address may have been used by spammers: to hide their own identity, they send spam emails that appear to come from you. They can do this by using your email address as their 'Reply To' address (this is called 'email spoofing').