Achieving success with email marketing relies on both art and science. The "art" element comes in the form of creativity, while the "science" element comes in the form of understanding which of those artistic elements actually work and having the empirical data to prove that it did. Unraveling the mysteries of email marketing requires a solid understanding of the factors which define and influence success. Let's take a look at a few now.

Unique Opens: Calculated as the number of addresses which were tracked as having viewed a message divided by the total number of HTML messages sent. To track open rates, HTML emails typically contain a 1x1 pixel image (sizes vary - Listrak for example uses a 1x20 image). When recipients have images enabled, servers are able to track when the image was displayed and by whom. When images are blocked, the "open" cannot be tracked. Open rates are becoming less accurate as people increasingly read email from hand held devices which often disable image downloading. 

Influencing Unique Opens: The biggest factor in determining whether a message is opening is your subject line and your "from: address. To increase opens, include your brand name to help increase recognition and differentiate the message from SPAM. Another tip is to provide "welcome messages" which include instructions on "white listing" your email address which increases the chances of delivery into their inbox, not bulk email folder.

Click Rate: The click rate is how many times the links in an email message were clicked. This is determined by dividing the total number of unique clicks by the product of the number of links within a message and the total number of recipients. For example, a message sent to 100 people with 5 unique links that get 10 clicks would have a 2% click rate (10/(100x5)=2%.

Influencing Click Rate: While it should come as no surprise that well-designed emails generate a higher click-rate, there are other ways to boost the click rate. For instance, including fewer links in an email will automatically (although arguably artificially) influence the click rate. Having an understanding of what readers will respond to and featuring clear call to actions (buy now, read more) also positively influences click rates.

Optimal Day To Send: No discussion in email marketing would be complete without a mention of what is the best day to send an email. Responses from pro email marketers vary widely, from early in the week, to the weekend. What's most important to consider however is the audience itself. A restaurant review website might do better on a Friday afternoon, than a message from a computer wholesaler on the same day. But the computer wholesale might do much better on a Tuesday than the restaurant website - so what's the answer? Your audience is the one that will help you determine which is which - unique opens and click rates speak at a greater volume than a email expert ever could.

Subject Line Length Matters: It may be the last thing that you write before hitting that send button, but it's an important factor in improving open and click rates - so take it seriously. Shorter subject lines (ideally less than 35 characters) have been shown time and time again to result in higher open and click rates, so while you may think that more is better, the opposite is actually true. As always, avoid those "ignore" triggers such as terminology including the much ballyhooed "free", the overuse of exclamation points and even currency characters such as the dollar or pound sign.

The Power (or lack thereof) of Personalization: By simply including the recipients name within the body of the message or the subject line, personalization clearly affects open and click rates. However, the results are often a mixed bag. If the baseline is not personalizing emails, then including a personalized subject line decreases both open rates and click rates. Personalizing only the message increases the open and click rates and doing both increases the open rates but decreases the click rates. See why it's a mixed bag? For now, best practice may be to personalize messages only when essential to the message and only within the message itself, not the subject line.