I hate to say this bellzy but, It could well be that your email address has been compromised.
This can indicate that someone has gained access to your email and is using it to spam other emails. I would suggest resetting your password ASAP and checking over the settings of the email to see if any email forwarding has been placed.
If taking those actions doesn't remedy the situation you can report this sort of spam from a BigPond service to Telstras so they can investigate. Just go here and follow the prompts and information required: https://service.telstra.com.au/customer/general/surveys/Report_misuse_of_service
Jeff