regexp looking for email addresses in iRule

I'm not sure what your ultimate goal is, but here is a very simple example rule that will scan for something (like an e-mail address) in a connection and reset it if found (however, it will not find strings that span TCP packets, but this should not be a problem with most SMTP clients):

 
 when CLIENT_ACCEPTED { 
    TCP::collect 
 } 
 when CLIENT_DATA { 
    if { [TCP::payload] contains "joe@foobar.com" } { 
       reject 
    } else { 
        Release the current payload onto the server 
       TCP::release 
        But still collect the next data from the client 
       TCP::collect 
    } 
 }