This are the sending IPs address from Redtail in case you have a customer using Redtail, you need to add the following IPs in the Sending Server
65.74.131.101
65.74.131.102
65.74.131.123
65.74.131.121
65.74.131.97
65.74.131.98
65.74.131.99
65.74.153.215
65.74.153.194
65.74.153.195
65.74.153.196
65.74.153.218
65.74.153.193
65.74.131.119
65.74.153.214
65.74.153.222
65.74.153.203
65.74.156.20
Redtail can create the Send Connector for the customer.