I have emailed many people about domains, buying and selling. I haven't really had anyone accuse me of spam although I have received a few kind-of rude "not interested" messages.
If they have their email address posted on their website and you use it to send them a direct email to offer them a domain you think they may be interested in, I think it is a legitimate business contact and not spam. If they complain, tell them that it was a one time email and you won't email them again...no big deal.