Right now, there is nothing that is preventing bots from spam signing up to a mailing list. There is a known scam strategy in which a scammer signs a victim up for tons of mailing lists, thus the victim might miss an email from their bank or PayPal telling them about a large withdrawal. That, and all the spam email addresses take up resources when sending out newsletters. I probably have 2,000 spam email addresses in my mailing list and I can't do anything about it!