Who knew I would be spending so much time thinking about spam filters this week?!
Well, I've found that I am most certainly not alone in this experience. The major problems seem to be with Verizon and Earthlink- but here's a whole discussion on the subject specific to Verizon. Apparently it's a very common issue... one that you never think about until it effects you, so my apologies to all those who don't have this issue but I'm trying to get to the bottom of this problem. I've emailed Verizon directly- but who knows if I'll hear anything back, I kind-of doubt it.
One of the recommendations was to open a new email account with someone like Gmail (it's free!)- not particularly attractive to everyone, but if it works for you it would be a quick fix and one that would save you from Verizon indiscriminately blocking your incoming mail.