Logging outgoing mails in debian? How, where?
Solution 1:
Normally, your Mail Transfer Agent will log the outgoing messages. Exim uses log files in /var/log/exim4. Log messages from other MTAs may end up in /var/log/mail.log. This assumes you are using a local MTA to do the delivery for you.
Running the command mailq as root may show a bunch of messages pending delivery. These may include some from the bulk delivery.
If the software does direct delivery, bypassing your MTA then you will need to see if the application has its own log file.