The good thing about using bulletins is that the email is available instantly to all users. No queues, no delays and a huge saving of system resources.
You can manually copy the file to /var/indimail/control/domain_name/bulk_mail directory or use the vbulletin command
% vbulletin -f /tmp/new_offers.eml -a example.com
vbulletin is especially useful when you want to target a subsection of your users or when you have large number of mail delivery hosts in an IndiMail cluster.
Read the man page vbulletin(1) for more details