From Seo Wiki - Search Engine Optimization and Programming Languages
Bulk e-mail software is software that is used to send e-mail in large quantities.
It might be used for legitimate mailings - for email list subscribers. Since the bulk email software usually sends email via direct send and via SMTP server the use of a bulk email software for spam is almost impossible, because of the large amount of emails that are sent in a spam campaign.
Bulk email software usually refers to a standalone software, while there are bulk email sending web based services as well.
A type of bulk e-mail software, sometimes referred as a mass mailer, are those computer worms that spread themselves via e-mail. Such worms usually (but not necessary) send spoofed From headers.