same problem here: NOT CONNECTED; 503 code, service not available,
this problem started a week ago (probably after the hosting provider upgraded to wp 4.7.3). i was on plugin version 4.0.13: posted same massage down at the other reports 4-5 days ago.
hoped the upgrade to plugin 4.1.0 would have fixed the issue, but not …
the mailchimp service IS AVAILABLE => service responded when tested using curl per mailchimp docs !!
i shall start looking for alternative. any recommendation anyone ?
Thanks