E-mail marketing can be sub-divided into 3 categories
1. Direct emails
2. Emailing E-zine / newsletters to opt in lists
3. Viral marketing
The primary reason for collecting visitors email addresses is so that a pro-active, cost-effective and continuous email marketing campaign can be carried.
Email marketing is one of the the most powerful tools available for marketers to do this. But like it's snail mail counterpart reponse is conditional on the integrity of the offer, creativity and lists. Generally speaking companies are advised against mass emailing as it is considered bad practice.
For the corporate user, where the systems are more likely to have broad band connections, security is in place to exclude mass emails and attachments.
However, campaign dependent, it is possible to buy targeted email lists where the recipients have agreed to recieve emails (opt in lists). If you use opt-in lists you're generally OK when you send emails... but ALWAYS add an unsubscribe function to the mails you send out.