Ever since I started working as a spam analyst, there has been an abundance of people that I know that write the word as "SPAM" instead of "spam." At first I thought that it was simply annoying. Now, I know that it is technically incorrect.
According to Wikipedia, SPAM is a canned, pre-cooked meat product sold by the Hormel Foods Corporation. It's proper trademarked name is SPAM, all in upper-case. You can take a look at that link to see a picture of a box of spam.
By contrast, plain, old spam is unsolicited commercial (or bulk) email. It is not in upper-case, it is lower-case. No trademark infringement and it's not an acronym. It's the conventional name given to the email annoyance everyone with an email address has to deal with.
Glad to get that off my chest (again).