Need help stopping exchange server spamming -- 2

Job ID: 35208164

Budget: $250 – $750 USD

I need help with my exchange server which suddenly started spamming and have been unable to figure out how they are getting in.
Need someone who's very good at finding the cause of this because so far 3 people have failed to find the cause

Notes:
SMTP is disabled in firewall
All firewall's are running on the servers, and all servers are running ESET Anti-Virus
Related categories: System Admin Web Security Microsoft Exchange