The 'key ingredient' in email marketing is having a quality email list – 'quality' being the operative term.
This means having a list of emails that you have attended by launching marketing campaigns, typically via a reputable email marketing service (like GetResponse or Aweber), with specific topics. In other words, these are lists of people that you know are really interested in the specific topics of your campaigns.
Conversely, quality email lists are usually not the ones you can buy online. Why not? Well, can you really trust the person that is telling you that the emails they are selling you are people basically interested in eg gardening or whatever other topic you are involved with?
I remember in earlier days of eBay, there were email lists on sale with supposedly millions of emails, that could be bought for under one dollar – how valuable do you think they were? The old adage 'you get what you pay for' may even have been an overstatement in this case.
And usually you could recognize the scam immediately for what it was, after buying these lists, by the mere fact that there were usually not 'millions' but only a few hundred emails (if even that many).
Another reason that purchased email lists do not work, is that the people you contact using these lists, do not know you, are not expecting to hear from you, have not given you, personally, permission to contact them and then will most likely not recognize you, trust you, or click your email (if it even reaches their inbox and is not weeded out by the increasingly effective spam detectors). Furthermore, what you are doing is called 'spamming' and, guess what, it's illegal!
To make a more general point that also applies here, in online marketing you need to know as much about your audience as possible. Your efforts need to be 'targeted', tailor to your potential buyers. You need to have an idea about what they want, what part of the world they live in etc.
Two email addresses may represent two people from very different parts of the world, with vast socioeconomic differences and buying capacities, very different age groups etc. Would not you want to know which email correspondents to which?
Just as an example, the toy industry, a multi-billion-dollar industry, expended great resources in order to figure out how to market to a part of the population that has zero purchasing power (children). And they succeeded by inventing ingenious ways to bypass the children and target their parents, by introducing the so-called 'nagging' effect.
Again, I mention that just as an example of the power of specificity and targeting in your marketing efforts, which naturally also applies to your email list. You absolutely need to know how the emails on the list were collected, if they correspond to people genuinely interested in a specific topic (again, do not take a stranger's word for that or anything else pertaining to your valuable business), the financial fortitude of these people etc.