alert tcp $SMTP_SERVERS 25 -> $EXTERNAL_NET any (msg:"GPL SMTP SMTP relaying denied"; flow:established,from_server; content:"550 5.7.1"; depth:70; reference:arachnids,249; reference:url,mail-abuse.org/tsi/ar-fix.html; classtype:misc-activity; sid:2100567; rev:12; metadata:created_at 2010_09_23, signature_severity Unknown, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2019_07_26;)