Mailchimp advise their delivery IP addresses in the form of CIDR ranges. These IP addresses may be used by recipients to whitelist Mailchimp sending IP addresses which will ensure higher deliverability of email campaigns.
The following are the IP addresses used by Mailchimp to send email campaigns as at February 2017. Please see the attached file for a list of all 86,017 individual IP addresses..
4096 IP addresses
IP address range: 126.96.36.199-188.8.131.52
IP address range: 184.108.40.206-220.127.116.11
16384 IP addresses
IP address range: 18.104.22.168-22.214.171.124