alert tcp $EXTERNAL_NET any -> $SMTP_SERVERS 25 (msg:"ET DOS Excessive SMTP MAIL-FROM DDoS"; flow:to_server, established; content:"MAIL FROM|3a|"; nocase; window:0; id:0; threshold:type limit, track by_src, count 30, seconds 60; classtype:denial-of-service; sid:2001795; rev:9; metadata:created_at 2010_07_30, signature_severity Major, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2019_07_26;)