Bulk email software allows you to send your ads to a large number of recipients at one time. For most senders, you can easily upload your email lists, no matter in .txt, .doc, or .xls format.
Aside from sender, there are other email software, such as email list management and email verifier. The former can help categorize your email addresses by domain name, country, and username. The latter can help keep your lists fresh. Email verifier does not send actual message to recipients. Rather, it only tests whether the address works, and can receive message.
Also popular is the email extractor. As its name suggests, such software can extract email addresses from an existing web page or crawl the internet to collect from various sites. It works something like search engine.
Of course, email address collected in this way is not permission based. Be careful before you send out any advertisement.