We send a monthly newsletter to clients. We would like to start breaking the list up into segments to target customers who have bought certain products and brands. If I create a segment for everyone who has bought Brand A and send it off, and then create another segment for everyone who has bought Brand B, I want to add another requirement to the second segment to exclude people who have been sent a newsletter in the last x days so that people who have bought Brands A and B don't get two newsletters. This would be easy if there was a field per subscriber for "date of last e-mail send".
Is this built in, or do I have to do some fancy work with the API in between each send?
This isn't built in, but thanks for the suggestion. I've added your vote to our list, we've had a couple of similar suggestions in the past.
Right now you would need to use the API and check that on your end.