Some problems in common definitions of 'spam email'
The most common attempts to define 'spam email' is either as 'UBE' (Unsolicited Broadcast Email) or 'UCE' (Unsolicited Commercial Email); for example, the spamhaus.org definition here. I tend to think that this sort of definition of spam has some problems.
Let's start with a provocative question: is advance fee fraud (so-called '419' email) spam email? (You know this type of spam; the classic version has Mrs. Mariam Abacha, wife of the late Nigerian dictator Sani Abacha, asking you to help get her husband's fortune to safety.)
A peculiarity of advance fee fraud email is that the messages are often composed by hand (sometimes by people sitting in an Internet cafe in Nigeria) and sent to relatively few people. So it isn't necessarily UBE (or at least not straightforwardly).
One can say that this is UCE because it is 'commercial' in the sense of 'having profit as a chief aim' (cf this definition of 'commercial'), but I think that this is stretching the term. The sender hopes to profit not through a business transaction with you, but by defrauding you out of some money.
But let's go one step further. Take a message that was dumped into my mailbox in August 2005, that started with:
The forgotten facts in all religions are explained by Allah through Imam Iskender Ali MIHR.
This is clearly not UCE; there's no attempt to profit, just proselytize (for www.mihr.com). This particular example was probably UBE (but I don't know for sure), but sooner or later similar messages may be composed by earnest people in Internet cafes and sent out just to you. Does that make them not spam? I'm pretty sure most people would disagree and call such email spam.
Clearly people's practical, gut definition of email spam is wider than just UCE or UBE.
Spamhaus has a technical definition of spam that would include the 'Iskender' email above, because it had no personalization for each recipient. But what if the earnest young men start personalizing their proselytization, perhaps using information from your web page; is their email transmuted to 'not spam' just because they are doing research and typing things by hand?
Was it spam when a fire and forget Microsoft recruiter sent Eric S. Raymond (a well known open-source booster and no fan of Microsoft) a recruitment pitch? (It was probably sent by hand.)
This matters because there are a number of ISPs and other organizations that find it convenient to define spam as only UCE (or UBE, depending on the organization). If their customers are doing things that fall outside of UCE or UBE, you are generally out of luck. (And I'm sure that Microsoft would assure us that the email to ESR is definitely not spam.)
Perhaps this is why brinkster.com has yet to do anything about www.mihr.com (IP address 126.96.36.199), despite the August 2005 spam being sent from mihrfoundation.com (IP address 188.8.131.52 at the time, right next door). After all, it wasn't UCE.