Know-How Exchange : Research/Metrics

If you have some money to spend, this is the email marketing benchmark report that the industry uses:

Sherpa

They just released the 2009 edition. I reviewed the 2008 version.

For some free numbers, try:

EmailStatCenter

Two email marketing services also release freely-available open and clickthrough data:

MailerMailer
MailChimp

One important thing to bear in mind is that these benchmarks are all averages across different email senders. So are not directly comparable to a particular, unique email program and audience.

Although you can get a rough idea of whether you're doing particularly badly or particularly well, the only benchmarks that really count are your own and whether you're improving them over time.