If you’re finding that people are accusing you of being a spammer, this may be your answer: your web-based email account. According the Australian Computer Emergency Response Team (AusCERT), spammers could use a form that allows you, the user, to e-mail feedback to the folk running your email service to send spam in your name. Security Wire Digest, a newsletter, quotes AusCERT as saying that “on some sites it’s possible to alter this destination address to anything an attacker wishes. In this way, third-party Web sites can be used to send spam to arbitrary recipients. Many of these sites also allow attackers to specify the body of the e-mail message.”
In short, a bit of a scam. If you think it may be happening to you, check with the people running your webmail service.