e-mail list decay rates

If you are building a permission list of emails - use it straight away, because it will decay over time. If the emails are old the first time you use them, fewer will get delivered, viewed and clicked than if they are fresh the first time you use them.

The longer you leave emails after collecting them, the lower the delivery rate falls. As a rule of thumb I found that you lose 5% of the whole list every 3 months.

Fresh emails (0 to 3 months old) are each worth three times as much as older emails (12 months old).

To find out exactly how much value was lost, I measured the value of emails by looking at our clients' UK B2B and B2C email campaigns. I looked at mixed lists of business and consumer emails collected by visitors completing client web forms voluntarily.

I knew the age of each e-mail (since collecting the email) and I knew the effectiveness of the email, measured by delivery rate and clickthrough. The value was calculated using delivery rate, email views and clickthroughs. My own simple formula is VALUE = (D x V x CTR x 100). For example:

email freshness = 0 to 3 months
Delivery rate = 90%
Views = 35%
Clickthroughs = 36%
VALUE = 11.34% (values around 12% are good)

email freshness = 12 months
Delivery rate = 73%
Views = 31%
Clickthroughs = 18%
VALUE = 4.07% (values around 4% are poor)

Summary: if you are collecting emails, use them quickly. Leave them for 12 months and you will get one third of the value you will get if you use them within 3 months.