Is that your first guess?
Spammers need to deliver spam messages, how do they do it? First, spammers have long lists of known email addresses. However, that’s only the first step. A spammer can try to deliver spam messages via his/her ISP’s SMTP server. This will fail very quickly because the ISP will notice the sheer volume of messages sent from a single client.
So, a spammer cannot rely on the SMTP server of an ISP to deliver all the spam messages.