The recommended way of doing this is to white-list the whole doodle.com domain, and configure your mail solution such that it checks the SPF records to determine if a host is a valid sender. This way you don't have to modify your configuration whenever we change our mail delivery setup.
If your mail solution does not allow this, you can still white-list our IP address ranges:
- IPv4: 184.108.40.206/26, e.g. 94.230.220 and 94.230.221
- IPv6: 2a02:418:6057::/48