Spoof, Spam & Blacklist

Bad Guy(s) spoof Company ABC email and launch spam on members of
popular search engine and online email provider (Company DEF)
Spammed DEF members reported the spam
Company DEF respond by looking up Company ABC's IPs and blacklisted
them (instead of offending IP address(es))

How do you get Company ABC off DEF blacklist?
How do you prevent Company ABC from getting blacklisted again?