From Wikipedia:

Bulk email software is software that is used to send email 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 email software, sometimes referred as a mass mailer, are those computer worms that spread themselves via email. Such worms usually (but not necessarily) send spoofed From headers.