[sleepydog]

Indeed. I used to work for another big mail sending service, and you would not believe the amount of effort that goes into maintaining good deliverability.

There was an entire team called "ISP relations" whose sole job was to work with big ISPs when one of our IPs get blacklisted.

There was another team that would interview customers after their sending passed a certain threshold to see if they were following best practices. They would ask questions like

* How did you collect your e-mail list? * Did your users give explicit approval to receive list e-mails? * Did you acquire this list indirectly, from another company/campaign? (big no-no).

Usually everything was fine, or the customer didn't know they were doing something wrong, and they corrected it. But not infrequently a customer (who pays a decent chunk of money/month) would be told they can no longer use the product. Besides it being the "right thing" to do, having recipients block or report spammy senders worsens the reputation of IP addresses.

An interesting exercise would be to calculate the number of man-hours put into maintaining the reputation of each IP address.