Another type of electronic mailing list is a discussion list, in which any subscriber may post. On a discussion list, a subscriber uses the mailing list to send messages to all the other subscribers, who may answer in similar fashion. Thus, actual discussion and information exchanges can happen. Mailing lists of this type are usually topic-oriented (for example, politics, scientific discussion, health problems, joke contests), and the topic may range from extremely narrow to "whatever you think could interest us". In this they are similar to Usenet newsgroups, another form of discussion group that may have an aversion to off-topic messages.
Transactional emails are usually triggered based on a customer's action with a company. To be qualified as transactional or relationship messages, these communications' primary purpose must be "to facilitate, complete, or confirm a commercial transaction that the recipient has previously agreed to enter into with the sender" along with a few other narrow definitions of transactional messaging.[2] Triggered transactional messages include dropped basket messages, password reset emails, purchase or order confirmation emails, order status emails, reorder emails, and email receipts.
Discussion lists often require every message to be approved by a moderator before being sent to the rest of the subscribers (moderated lists), although higher-traffic lists typically only moderate messages from new subscribers. Companies sending out promotional newsletters have the option of working with whitelist mail distributors, which agree to standards and high fines from ISPs should any of the opt-in subscribers complain. In exchange for their compliance and agreement to prohibitive fines, the emails sent by whitelisted companies are not blocked by spam filters, which often can reroute these legitimate, non-spam emails.[3]

…email blasts are, frankly, inconsiderate from a recipient’s standpoint. We define spam as any unwanted email, and most of the time irrelevant emails are unwanted. Email blasts by definition are irrelevant since you can’t possibly send the same exact message to hundreds of thousands (or even millions!) of subscribers and expect it to resonate personally with each one.

The call to action doesn’t have to be a sale or discount – it could be advertising a contest or promotion, or incentivizing people to conduct an online survey. Regardless of the nature of the call to action, there must at least BE a call to action. Direct mailing is most certainly not the avenue to send out a blank postcard or flyer that simply informs people of your presence – it’s too expensive, too time consuming, and too hard to efficiently track metrics to garner simple impressions.

If you want to operate an email blast campaign as a part of your marketing strategy, then you need to send the emails out regularly. If it is too long between emails, then the recipient might not even remember what your content has to offer. However, you do not want to send them too frequently. If the consumer is getting an email from you every other day, then they might get annoyed and stop reading altogether.

There are many vendors out there who sell lists or rent them (though renting means that the list seller maintains ownership and control of the email list). These are collections of email addresses that the vendors sell to any business or individual who can pay the fees. Your email list is considered to be a purchased or shared list if it’s provided to you by a third party, like an email list vendor or affiliate. There's a few ways that vendors build these non opt-in email lists.