Why Buying an Email List is Bad
Most business owners have good intentions when they purchase an email list. They're often quite eager to grow their business and feel these lists will help them grow faster. There are companies out there looking for people new to email marketing and they promise to deliver lists that are "cleaned", "subscribed/opted in", "verified" and "targeted". The truth is sending email to a purchased list has consequences. Here are some consequences you might not be aware of:

ISP’s may blacklist/block email coming from your email address 
ISP's can choose to blacklist/block your "from" email address due to a high number of emails sent to invalid email addresses. Purchased lists normally include a high number of invalid email addresses. Once an ISP blocks you even contacts with valid email addresses will be blocked, for that particular ISP. There's also the possibly that one of the email addresses in your purchased list is a "spam trigger". Which means anyone sending email to that specific email address will be blacklisted/blocked on that particular ISP because they sent email to a "spam trap" email address.
Note: An invalid email address is an address that cannot be delivered to the recipient because the email address never existed or doesn't exist at the time you sent the email.

Increases the chance of your email being marked as spam
Recipients from a purchased list may mark your email as spam because they never subscribed to receive your business' newsletter. If a large number of contacts mark your email as spam you can be blacklisted/blocked by an ISP.

May hinder you from sending email to contacts who actually subscribe/opt-in to receive your newsletter 
As mentioned above once an ISP blacklists/blocks your "from" email address even contacts with valid email addresses, for that particular ISP, will be blocked.

Negative company reputation
All of the above can cause your business to have a negative reputation within email marketing.

Also, just uploading a purchased list into NCR Customer Connect has it's consequences

Anytime a list is uploaded into NCR Customer Connect, using the Manage/Lists tab, it checks the list to see if the email addresses are valid; prior to you sending email to these addresses. While checking your uploaded list, all email may be delayed until the list is completely checked. If the list was purchased, the checking process may take longer due to the number of invalid email addresses; which is common with a purchased list. Please remember the terms and conditions on the Manage/Lists tab specifically states, "I attest that this list of email addresses has not been purchased, rented or obtained from any third party". Checking this box means you agree to these terms and conditions.

Review 10 Steps to a Healthy Contact List for ideas on how to build "good" lists.
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