By this definition John sending an unexpected party invitation to his friend Dave is spam.

edit: I guess spam is by definition commercial email. Still, I think it's possible to have a business relationship with a company where it's acceptable for them to send an occasional email that you didn't specifically request.

Business relationships are different from personal relationships, and "solicit," in this case, does not mean literally request the email. It is perfectly reasonably to describe unexpected email from a friend as solicited and unexpected product email from a business as unsolicited (even if you've had contact with them previously).

It is unlikely that the business is your friend ;-).

Personally, there are a select few brands from whom I enjoy reading unsolicited emails.

They have proven themselves to me and I like them on an irrationally human level.

What bothers me, however, is when the other 99.9% of the brands I interact with automatically assume that they are the 0.1%.

Yes, it's possible and acceptable to have that kind of relationship between a client/business, but the problem becomes when a brand believes that they can control that relationship, or define it themselves, or simply assume that it exists.

"I guess spam is by definition commercial email"

That's only some definitions. There are more hardcore people who'd view anything they didn't explicitly request as 'spam', just some of it originating from people they know vs companies they know.

What about solicited mail? Sure, unsolicited newsletters are annoying, but you're making it sound like NOBODY, EVER subscribes to newsletters. Sometimes, someone (who is not you) subscribes to a newsletter, and then after a period of inactivity s/he receives a newsletter but does not immediately recognize the title, and hits the Spam button out of habit. I think this is what _asciiker_ is talking about. What about cases that this?

I think you need to break down SPAM this way...

SPAM = UCE + UBE.

UCE = Unsolicited Commercial Mail

UBE = Unsolicited Bulk Mail

What's common? Unsolicited.

Definitions make a big difference here.

A week or two ago I sent hundreds of messages to my customers, warning about Heartbleed and noting what measures we'd taken against it.

That mail was unsolicited. Not overtly solicited, anyway. Sure, it falls under the "we may contact you from time to time about yadda yadda" but people only know that if they actually read our Terms & Conditions. A normal person with an important mailbox and a convenient spam-button doesn't always have time for such careful consideration and nuance.

Heck, to make sure the mail reached the recipients, and to avoid getting my domain flagged (and this is for a very legitimate message!) I used a 3rd-party mail service. It sucks that I even need to do that just to communicate with my customers.

I suspect one or a few of my customers may have marked that message as spam, too. Because, whatever it was (didn't read, just glanced at subject) they didn't expect it so it must be spam.