Default Mailchimp API issue

Hi guys, I am trying to integrate the default E-mail marketing with Mailchimp and I am getting an error. I have inserted the Mailchimp account key and my subscriber list is reflected on Cs-cart, but when selecting the list and hitting save I am getting this error:
" Error We couldn’t verify the URL is working. Please double check and try again. HTTP Code: 503"
What amI missing?


Please contact us via Help Desk on this case, I was unable to reproduce this issue using my test installation.