What version of SBG are you using?
Define "too many messages"
If you use an external DNS source, you will not be able to resolve internal hostnames or private MX records within your network. You'd need to specify IP addresss instead of hostnames for all your local domains. SBG 9 allows multiple IPs in more places. If you are using DNS round robin (host name points to multiple IPs) for load balancing this could be an issue.
It looks like your internal DNS is not effectivly caching for the volume of e-mails you are receiving. Try turning off "Enable reverse DNS lookup" on your scanner(s) under Admin / Configure / Scanner name / SMTP / Advance Settings / Inbound tab.
This will stop SBG from resolving the IPs of all the spammers talking to your scanner(s).