Prevent blacklistings

We use google for email.  I have blocked smtp at the firewall on our network.

We recently just got blacklisted again... not sure how yet.... I will be looking into that... but in the meantime I am trying to figure out if this really needs to happen.

So say we have a spambot on our network... why should people be refusing email from google's servers?   It's not as if the spam bot on our network can send mail from googles servers.  I assume this probably has something to do with the fact our domain name has subdomains that point to the firewall for our network... and so it's being blocked because the domain on the email matches the domain tied to an IP that someone saw some spam from.  

I don't know a lot about it, but this seems excessive to me.  I mean, sure, block email originating from the same IP as the spam... but if it comes from an IP (i.e. google) that you don't see spam from, them why block it?  

In any case - is there something I can do, DNS wise, to prevent a spam bot on our network getting use blacklisted such that people are refusing our email send from google servers?  (Granted... I know we need to get rid of the spam bots.... but not interrupting our perfectly safe business email in the meantime would be preferable)