alert tcp $EXTERNAL_NET any -> $SMTP_SERVERS 25 (msg:"GPL SMTP expn decode"; flow:to_server,established; content:"expn"; nocase; content:"decode"; nocase; pcre:"/^expn\s+decode/smi"; reference:arachnids,32; reference:cve,1999-0096; reference:nessus,10248; classtype:attempted-recon; sid:2100659; rev:10; metadata:created_at 2010_09_23, cve CVE_1999_0096, signature_severity Unknown, updated_at 2019_07_26;)