It is almost certain that your address (along with a great many others) was obtained from an unscrupulous service provider, and was used as a forged sender address to send out spam. It is extremely unlikely that your own account was actually used to send the original message unless you have disclosed your username and password (knowingly or not) to a third party - see FAQ 1446.
HOWEVER, as it is still possible that your account may have been used by an unauthorised person, it would be a very wise precaution to change your password immediately.
Forging sender addresses is a very common practice amongst spammers. In such circumstances it is inevitable that you will receive at least some replies or bounces to such messages, whether generated automatically or not. There is almost nothing you can do to stop this with current technology, so it's best just to "ignore, delete and forget" any such messages.
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This is question number 1193, which appears in the following categories: