When we started sending e-mail newsletters six years ago virtually all of them got opened. There was a big difference between spam and the mail you wanted to get. The middle ground has filled up.
Now the average person receives 72 emails each day and opens less than 20% of them. Couple this with the projection that in 4 years the number of emails per user is likely to double.
You don't need to be a rocket scientist to realize that you will have to compete for all of those opens. You can't spend your advertising budget on emails that don't get opened. Manage your content to build your open rate, just like you build your subscriber list.
We watch these open rates very carefully. We strive for an open rate greater than 60%. When we start with a clean list, we keep it over that number in almost every case. In some cases the number is lower because the lists are dated, or have been spoiled because a sender didn't worry about the content. We have taken lists over after a nursery used a mailing list to send virtually the same sales flyer month after month. Without useful information, fresh ideas, and a bright attitude, those lists became cold. It takes a long time, and some great marketing to rehabilitate those lists.
One major factor is send frequency. We were surprised to see that the highest open rates happen at "off gardening times." January and February have the highest open rates of all the months. If the weather isn't suited to garden then people read about gardening. J
Another contributing factor is how long since the last newsletter was sent. To our surprise, emails that follow 2 weeks behind the last email open 9% more often than monthly emails. The like to make sure that every email contains some treasure, and that there are offerings across a wide audience. You should offer something for the novice, and something for the expert. We all know that the widest opportunities are with the broad center of casual gardeners who would like to be just a little better.
If you want to consider an effectiveness formula, it would look something like:
(# of subscribers) X (open rate) X (frequency) X (average purchase) = newsletter value.
I would love to hear some differing opinions, or suggestions. Happy retailing!