Ellen Paul at Paul & Partners does a good job of explaining problems and solutions for mailing list hygiene in her "Dear upset client…" article.
I don't know if her story is a true one. If it is, it's a sad story of a house file with 79.4% bad data. But I am guessing this is a fictional compilation of true stories from various clients over the years. It's a good summary of how a mailing list can be neglected and how to fix it.
Think about the postage and printing savings and ROI improvements by keeping your mailing list clean.