Heck, I write e-mails based on the assumption that people don't read them. It is sadly quite common for people whose sole job it is to read and reply to my e-mails (e.g. customer support) to read only the first few words of the first few paragraphs, generally stopping after the first noun and verb in each.

I find that those are the same people who tend to write content-free walls of text. No wonder they only read the upper-left-hand corner of e-mails, that's the only part of what they write that's worth reading.