When using email marketing do not use bulk email senders, your emails may end up in spam, but use OE or outlook or eudora or such products to send emails 10 or 20 emails at a time, so the emails reach the mail inbox.
There is no much difference if you use outlook or other tool. both could use the same SMTP and from recipients side it is the same.
Have you tried this and been caught out? Spamming is obviously different from sending out emails to your subscribed list...
I thought it depends on what the email looks like. what if you said: "Hello, have a wonderful day!" Would that be considered spam if you sent it out to 1 million people.
if they are not opt-in then it is spam check this: http://www.ftc.gov/bcp/conline/pubs/buspubs/canspam.shtm