Never use a list acquired from a third party
Sending bulk emails to large mailing lists can be a effective low cost method of broadcasting your message. However, you should take great care to closely guard where you get your contacts from. The internet is full of promises to provide you with targeted mailing lists for your industry or sector.
Our advice is that you should never acquire a list of email addresses from any third party, however reputable they may appear. At best, it is likely to be out of date and often the original source of the list is unclear. At worst, the list is seeded with secret email addresses that alert SPAM blocking services that you have acquired the list from an unreliable source. What then happens is the server you are sending your emails from (and possibly your domain name) will get blacklisted.