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Detecting BCC's with a SMTP event sink

A couple of weeks ago I blogged this about BCC's and CDOEX, One of the questions that comes up when you start to deal in BCC's is can you detect a BCC programmatically for instance in a anti-spam event sink or if you are worried about confidentiality in an email that was sent. The answer is kind of. For instance you could implement a SMTP event sink on your forward facing Exchange /SMTP box that could detect if a email comings into your organization was being BCC to anyone in your domain. You do this by using the http://schemas.microsoft.com/cdo/smtpenvelope/recipientlist envelope field. This field will contain a list of all the recipients of a message that was submitted by the client (or sending mail server) before expansion of the address has been performed (or categorization in Exchange speak). On a inbound message this would only be the recipients that where in your domain that the SMTP server could deliver to. If you where to compare the recipientlist to the to and cc fields of the message and find any address's that are on the recipeintlist but not in the to and cc fields then you most probably have a BCC. Here's a sample on_arrival sink that would do this

Sub ISMTPOnArrival_OnArrival(ByVal Msg, EventStatus )
on error resume next
Dim RecpList
recplist = LCase(Msg.EnvelopeFields("http://schemas.microsoft.com/cdo/smtpenvelope/recipientlist"))
recparray = split(recplist,";",-1,1)
Set fso = CreateObject("Scripting.FileSystemObject")
set wfile = fso.opentextfile("d:\RECPT.txt",8,true)
for i = lbound(recparray) to ubound(recparray)
if instr(msg.to,replace(recparray(I),"smtp:","")) then
else
if instr(msg.cc,replace(recparray(I),"smtp:","")) then
else
wfile.writeline("BCC Detected " & recparray(I))
end if
end if
next
wfile.close
set wfile = nothing
End Sub

Now this would also work on outgoing mail by finding any BCC's that weren't from your own domain (which would have probably been delivered and removed from the recipientlist). The place that this falls down is it doesn't detect BCC's that wheren't sent to another domain. Say if the message had been sent to you and BCC'd to a yahoo or hotmail account the Recipeintlist won't contain entries for the yahoo or hotmail account. (this would be different if the mail was going outbound from your server where it would contain those entries.)

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