Bulk email or mass email is when you have to send emails to a big list of contacts, that happens a lot in the email marketing and on newsletters.
There are two situations, one, you have already a huge list of contacts and you need to email to all, second, you are building a list, lets analyse the two.
Already have a list of contacts.
Well, probably you are a spammer or at least you are about to become one. if you are in this situation you may try a free software called EmailSender, it will do the trick, it works with free email providers such as Gmail, if you use it with Gmail you can send only 500 emails daily, so if you have a huge list you may need to pay for a email provider of your own, for more details on how to do it check my How to Send Bulk Email With Gmail hub.
You are building your own list of contacts.
This is the best option to not become a spammer and if you do it all by the the book you do it in a legal way, so your emails probably wont end up in the spam folder.
There are several free software to do this, PHP List is one of them, it must be implemented in a remote server, so if you have the skills you know what to do.
If you are running a Drupal or Joomla website there are several modules for sending newsletters just search the one that suits you better.
And finally, if you don't have any technical skills and just need an easy and cheap way to send your newsletters and email marketing campaigns you really should try AWeber, it's just $1 dollar the first month, and you don't need to worry about the law, AWeber does that for you.
Check my other hubs about Email Marketing and remember don't be a spammer.