SMTP Relay Works 50% of the Time

Here's our setup. 

We have two Exch 2016 servers, mailboxes for our office are split 50 / 50 between them. We have an app that sends email alerts internally with attachments through anonymous relay. We protect this with IP address authentication on both servers. I set the app to use the IP address of Server1. When messages are sent, they arrive in the mailbox of anyone on Server1. Anyone on Server2 does not receive them. If I set it to use Server2 IP then Server1 does not receive the mail. The internal transfer between servers does work, just not for this particular app. 

If I change the app SMTP settings to one of our overseas offices, which has a single Exchange 2016 server, the mail arrives on both Server1 and 2 (after a brief delay) without issue. 

Why would internal Exchange server to Exchange server transfer work, but the indirect mail relay is not transferred to the other server???

Thanks.