Know-How Exchange : E-Marketing

We look at email open rates as a gauge of interest for a specific email blast. I am losing confidence in this as a viable metric in that I see strange numbers. For example I see opens in the 30's. How can someone open an email 30 times. I more commonly see users opening email 5 or 6 times. I never do this with email - so not sure why others would open an email repeated times. Is this an accurate measure?