Hi everyone just got this back from Mailchimp:
" we would recommend checking with the subscribers to make sure that they have your "from" email address in their contact list or added to their address book. Email clients tend to allow email to be passed along properly if the subscriber has listed the reply-to address as a known sender.
If there is some type of firewall or filter that may be blocking emails from coming through, then we would recommend having subscribers whitelist MailChimp's servers. Since most corporate or educational email addresses have strong filters, whitelisting our servers should help things get through to the inbox."
Apologies if any of that sounds obvious. This is kind of a difficult thing to resolve since we can't check the email accounts of everyone ourselves.