I'm trying to track down email marketing statistics for travel
industry-specific email campaigns. This is fairly specialized type of
knowledge so this question is best suited to a researcher who already
knows something about online / email marketing. I'll use this data to
help establish benchmarks / goals for my company's own travel-related
email marketing efforts to existing customers who have actively opted
in to receiving mailings from us. The major metrics I need are as
follows:
- open rates 
- click-through rates
- unsubscribe rates
- conversion to sale rates (buythrough percentage)

A successful answer to my question will provide a detailed reponse,
preferably containing both an average percentage, or a % range, for
all the above data points (example: the average clickthru rate for a
travel opt-in newsletter is 20% and with a range of responses varing
from 10-30%) and include documentation of the source of your data,
including URLs where I can go and browse the research myself. That
being said, it maybe that there is not enough publically-accessible
data on these topics to provide
all I want, so, within reason, I'll take what I can get.

From my past experience I know that email response rates are often
segmented between mailings to existing customers (loyalty or retention
marketing) and mailings to non-customers (acquisition marketing). I am
most interested in stats about mailings to existing customers (though
data on travel-industry specific acquisition mailings would also be of
use to me).

Another success criteria for answers to my question that should be
stressed is that I need these metrics to be specific to the email
marketing of travel products (hotels, air, air/hotel packages, car
rentals, etc) to consumers. While metrics about overall email
marketing are definitely welcome, its the travel-specific data that I
really need.

Thank you for your time and consideration of my question. If YOU have
any questions about my request, please let me know.

Sincerely, 
Mike