Most bulk email software programs are hosted by third-party companies that sell access to their system.
[1] Customers pay per send or at a fixed monthly rate to have their own user account from which they can manage their contacts and send out email campaigns.
Generally the advantage of this type of program is the reliability of the third party vendor and their application.
The disadvantage of using this option is that the delivery rate is reduced as users frequently use one server to send bulk emails.
There are various settings to tweak to avoid a server being labelled as spam.