alert tcp $EXTERNAL_NET any -> $SMTP_SERVERS 25 (msg:"ET MALWARE Asprox-style Message ID"; flow:established,to_server; dsize:<80; content:"Message-ID|3a20|"; depth:12; content:"|0d0a|"; within:68; flowbits:set,ET.asproxmessageid; flowbits:noalert; reference:url,www.secureworks.com/research/threats/danmecasprox; classtype:trojan-activity; sid:2008221; rev:5; metadata:created_at 2010_07_30, signature_severity Major, tag Description_Generated_By_Proofpoint_Nexus, updated_at 2019_07_26;)