Getting on Top of Spam Down Under
The Register is reporting that Australia has implemented a new industry code for the regulation of email with respect to spam. From the article: "Under the new code, internet service providers (ISPs) will bear some of the responsibility for helping fight spam. Service providers must offer spam-filtering options to their subscribers and advise them on how to best deal with and report the nuisance mail. ISPs will also be compelled to impose 'reasonable' limits on subscribers' sending email."
You seem to be a business. As a business, it is up to you to decide what services to offer your customers and what to charge for those services.
It is cheaper to not do anything about the situation and just buy more bandwidth as you need it. That's a business decision you have to make.
If your customers are swamped in spam, that is also a business consideration for you. There is a chance that they'll leave and go to a service that offers everything you offer and offers some degree of spam protection.
If you're offering email services, you should at least be monitoring the outgoing levels and taking automatic precautions when there is a huge jump in outgoing volumn. Do NOT become part of the spam sending problem.