Bulk email software is software that is used to send email

Email

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

 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

Simple Mail Transfer Protocol

Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...

 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 worm

Computer worm

A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...

s that spread themselves via email. Such worms usually (but not necessarily) send spoofed From headers.