Testing and Debugging
To test the CDO transport event sink script, register it and send an email message to the computer that runs the SMTP service. For example, you could run Outlook Express and send a message to any user at the LocalHost domain (e.g., test@localhost). To configure the service to have a LocalHost domain, open the IIS snap-in. Expand the Default SMTP Server tree and select the Domains node. Right-click the node and select New, Domain. The New SMTP Domain Wizard will ask whether you want to create a Remote domain or an Alias domain. Create an Alias domain named localhost. Then, you can use that domain to send messages from the computer to itself.
Because the SMTP service has no UI that lets you watch the Perl script execute, debugging the script can be difficult. The easiest way to debug the script is to write log data to a log file, which lets you print the values of variables at different points in the script.
However, managing an ever-growing log file can be burdensome. A syslog-like script, such as the one in "Converting Perl Scripts to Win32 Perl Services," May 2003, InstantDoc ID 38404, can be quite useful. You can download syslogd.pl from the Windows Scripting Solutions Web site and use it as your log file. Run syslogd.pl in a command-line window and modify MessageFilter.pl to open the named pipe that syslogd.pl creates. To do so, modify the $LOG_PATH variable at callout B in Listing 1 to
$LOG_PATH = "\\\\.\\pipe\\syslog";
MessageFilter.pl will then open the syslog script's named pipe and print all logging information there. The information will appear on screen as it's logged.
A Winning Solution
In my experience, Perl 5.8 and Perl 5.6 both work well. However, Perl 5.5 seems to cause IIS to crash repeatedly. I suggest that you use the latest version of ActivePerl for MessageFilter.pl.
Using scripts for CDO transport event sinks can be a winning solution for your Exchange server. Because sinks are easy to code and quick to modify and prototype, you can have a message filtering solution running quickly.
MessageFilter.pl is in production on my network and has exhibited quite palatable performance while filtering out tens of thousands of messages sent by the MyDoom virus. You can easily modify MessageFilter.pl to better meet your needs. For example, instead of discarding email messages that have an offending attachment, you could remove the attachment, add an explanatory note to the message body, then send the message to its intended destination.
You could also write some interesting variations of MessageFilter.pl. For example, you could add scripts to do spam filtering (freeware—
and even Perl libraries—
are available for this work), intelligent message routing, virus checking, removing HTML code from email (or stripping out references to graphics and links), and censoring sensitive data. And when the next big virus du jour appears, you can quickly mitigate it on your Exchange server simply by adding a couple of lines to MessageFilter.pl.