Is it legal to send unsolicited e-mail to businesses under the CAN-SPAM Act?

I am still confused if sending unsolicited e-mails to businesses is illegal. I've read around the internet that sending unsolicited e-mails is legal as long as the headline is an honest one, there is an opt-out option, your business address is on there, and you mention in the e-mail that it's an advertisement. I guess the part that really concerns me is the unsolicited part. I see businesses marketing their services by e-mail to other businesses without having any prior business relationships with them, so these would be cold e-mails. Is this illegal period? Or is it legal to send these unsolicited e-mails as long as they follow the guidelines I have described, even if they don't have an existing relationship with the mail recipient?