This is funny, because I was just complaining about how some departments send out way too much email, much of which has nothing to do with me or my job, so I just get used to deleting them out of hand.
I'm not saying this is you, but for example, or IT department sends out these "URGENTURGENTURGENT" emails that are rarely urgent to me - for example, today they send out an email to the entire organization telling us that our network connection to a particular overseas office might be slow.
1. I don't care.
2. I so don't care.
3. I read half of it and deleted it.
The problem happens when one in twenty emails actually is important, but you don't read it, because the other 19 emails were useless to me. It's just human nature, especially since people are pretty overwhelmed by their email traffic. My email box, with frequent archiving, is often at the 100 MB mark. (My email archive is in the gigabytes.)