Approximately 5-10 times a day I seem to have a bot that goes to my website and tries to sign up a random email address to my newsletter. Each time it happens, my server sends out a "confirm you want to subscribe" email and they never ever click to confirm. Many of the emails bounce as they are sent to non-existent email addresses. I don't understand what this is supposed to achieve, except for maybe ruining the reputation of my email service.

It's always coming from the same country (the Netherlands) but never the same IP.

I'm wondering if other people's websites/newsletters are experiencing the same?