SMTP Sensitivity Tag

Well, that's a good start. My two main suggestions are:

A) You shouldn't need to use a global variable for ::tcpdata. Just remove the :: from the variable and remove your RULE_INIT event. It should work just fine with a local variable.

B) You should either fix your flow to return after matching a command or change to using elseif's or even better, use a switch command to structure all the different command matches. EG:

switch -glob $tcpdata {
"EHLO*" {
      TCP::respond "250-fdpnmailgw2.thehouseofspam.com Hello\r\n"
      ...
   }
"MAIL FROM:*" {
      TCP::respond "250 OK\r\n"
      set mailfrom $tcpdata
      ...
   }
"RCPT TO:*" {
      ...
   }
"DATA" {
      ...
   }
".\r\n" {
      ...
   }
"QUIT" {
      ...
   }
default {
      append tcpdata [TCP::payload]
   }
}