A dirty email list can be an Nigerian email list that's NOT had the duplicate and undeliverable email addresses deleted from it and moreover -- it includes lots of email addresses of people who do NOT want for solicitations.
More is NOT always better! A person thinking of buying a Nigerian email list will find the same Nigerian email list for sale on different websites. If the price is the same, they'll usually purchase the list most abundant in records. That is not necessarily the very best decision.