Invoice emails come from different ip addresses

[kvo]

VM 3.2.12
Why is it that emailed invoices (generated by new orders) come from all different ip addresses? We are getting the following notice on order emails:
"Sender failed our fraud detection check and may not be who they appear to be..."
We tried to add ip address of the host server, but it keeps changing.
I checked email header of 4 different emails and the ip address of who the emails are received from is different on all.
Any guidance would be greatly appreciated!