alert tcp $SMTP_SERVERS 25 -> $EXTERNAL_NET any (msg:"GPL SMTP AUTH LOGON brute force attempt"; flow:from_server,established; content:"Authentication unsuccessful"; offset:54; nocase; threshold:type threshold, track by_dst, count 5, seconds 60; classtype:suspicious-login; sid:2102275; rev:3; metadata:created_at 2010_09_23, signature_severity Unknown, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2019_07_26;)