I have asked this question in the past and was told it was not a concern. My host has locked down all my sites from sending emails because there is no Bot protection on the course Signup button.
Here is their statement:
"If I click the "Signup" button I am greeted with a form that lacks any kind of CAPTCHA image or other human verification method. This form is susceptible to BOT attacks which could signup unsuspecting users for your mailing list. This also explains the high rate of messages about new registrations we're seeing in our logs.
Please implement a CAPTCHA image so we can ask that our email administrators unblock the account. Thank you for your cooperation and patience with this matter."
I am going to need another answer than just installing Akismet.