If you send to a group of people at a client company, and they aren't getting the emails, their company firewall may be blocking them. Firewalls don't like it when multiple people at one company get the same email at the same time, and often treat those emails like spam.
Ask your client's IT group to whitelist MailChimp. Check out our whitelisting information for a list of the IPs we use for sending. This is all the information the IT team will need to in order to whitelist the MailChimp servers, so that future emails get through.
If you'd like to be notified when we update our IP addresses or servers you can stay up to date with the RSS feed.