Hi Rene, 

You can certainly use Message Sniffer as one of your outbound checks. Please note that any check that's enabled for Outbound SMTP Blocking must be used with the Outbound Weight Threshold that's found on the Antispam > Options tab > SMTP Blocking card. From there, you can specify a weight that should be considered "spam", then choose to Quarantine or Delete that message, and choose whether to send the user a bounce message about the delivery failure. This feature will catch standard deliveries that are considered to be spam. 

To prevent spam from being automatically forwarded, I would encourage you to set the domain's Forwarding Exclusion to Spam Low/Medium/High. When you edit a domain, you'll find a Forward Exclusion setting on the Email card. To prevent all spam probabilities from being auto-forwarded, set this to "Low and above". You can also adjust this setting in Domain Defaults, and propagate it to domains, as needed. 

Finally, SmarterMail offers a Do Not Forward list, where you can add domains that have strict requirements and are likely to blacklist you for spam, such as Yahoo, AOL, Verizon and Gmail. You can find this in Settings > Delivery Limits. 

I hope this helps!