This is one way to find public IP Ranges for email using DIG. This is used if you want to get the latest IP Ranges for whitelisting. The vendor might have this available on their site but if they don’t, this is one way to get them. You’ll notice you’re looking for the SPF configuration. Since SPF uses IP Addresses to help prevent SPAM, it’s one way to get the IP Blocks or just IP’s that are used for sending email.
1. DIG gmail.com txt = find SPF
2. DIG _spf.google.com txt = look at SPF
3. DIG _netblocks.google.com txt = look at IP Block
4. DIG _netblocks2.google.com txt = look at IP Block
5. DIG _netblocks3.google.com txt = look at IP Block