If you have a dedicated IP, you can instruct your clients or recipients to whitelist just that IP. If you don't have a dedicated IP, though, your emails will be sent from Mandrill's shared IP range. This range is continuously updated as we add more servers for sending.
Whitelisting by IP is not recommended because Mandrill's shared IPs may change without notice, and the recipient would be whitelisting all mail from Mandrill. If whitelisting by IP is the only option, the recipient domain will want to check for the most up-to-date IP information in the TXT record for
spf.mandrillapp.com. On a Mac or Linux, this information can be obtained using the following command:
dig TXT spf.mandrillapp.com
This record will be updated whenever new IPs are added to Mandrill's sending IP range. If a receiving server is whitelisting the range, it's recommended they periodically check to ensure they have the current range used by Mandrill.