A cracking article from Stefan Pollard for ClickZ on Sep 9 2009, called "Avoid Desperate E-Mail Tactics This Holiday" covers a very good overview of today's deliverability concerns and some of the causes.
It goes from old school spam reports to the latest plans for domain reputation, worth a read but it's quite long!
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On the subject of holidays, it is really easy to really bugger up your reputation as a sender, not just with the ISPs but with your recipients!
You want to really make sure that every one gets the best chance to buy from you but also you can overload them and scare them away.
Pure360 has a cheeky little check box called Contact Frequency Limiter. You can set it to only send an email to one address every X number of days and Pure360 sorts the rest out.
I don't use because I profile but if you are a generic sender this could save your list!