If you're sending to a group of people at a client company, and the emails aren't being received, it's probably their email firewall blocking the delivery. It tends to look suspicious when multiple people at one company are getting the same email at the same time and as a result the email can be seen as spam.
If this happens, your clients can ask their IT group to whitelist MailChimp. Check out our whitelisting information or our 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.