Foreign SMTP traffic being blocked at the ISP level. Home IP block ranges being blackholed. PTR records not provided to home accounts. All these issues are solved using a Smart Host. Unfortunately, depending on the monthly fees, it might be cheaper to just upgrade to a business class ISP account or host your e-mail elsewhere.
I've only had to setup Smart Hosts for clients with an SBS box that was in rural parts of America. Sometimes netblock ranges were blackholed and/or PTR records weren't supported at the ISP level for their DSL connection. And they were lucky to even have that for connectivity.