The recommended way of doing this is to white list the whole doodle.com domain and configure your email solution to check the SPF records to determine if a host is a valid sender. This way you do not have to modify your configuration whenever we change our email delivery setup.
If your email solution does not allow this, you can still white-list our IP address ranges:
149.72.180.54
168.245.95.11
168.245.78.192
168.245.10.81
168.245.120.5
168.245.114.108