People want offers. We're in the process of helping them clean up their mailing list at the moment, and the open rate is rapidly getting to the 30% range for ordinary offer e-mails and coupons, and much more for "special" offers (like 50% off vouchers).

Especially since the financial crisis, people have been very budget conscious, and a lot of people are on these mailinglists to get offers for lunch etc.

We actually have a nice timeline of the crisis: When Lehmans toppled, bookings at the restaurants of our clients near their building shot through the roof. And looking back at bounce stats, we can tell which firms had large layoffs when, as we suddenly had 30% of addresses at one city firm bounce, 50% of another, and so on.

Have you ever ordered pizza online?

I know some restaurants will offer birthday coupons for joining their mailing list, which might attribute to the large amount of customers on one.