Exchange 2019 ActiveSync internal network native iOS Mail not working

Im in the process of migrating from Exchange 2013 to 2019. During this process I changed my routings (mail flow, OWA,...) to the new servers.

Public-Routing: Public DNS Record --> Azure Trafficmanager --> public ip adresses of my exchange --> NAT to the Exchange Servers. ActiveSync works perfect.

Internal Routing from Guest Wifi: Its a seperate VLAN which allowes access to the internet. The Internet Relay has a static entry for the OWA/ActiveSync-URL which points to the internal IP-Address of the exchange server(s). The Firewall is the DNS-Server in the guest wifi. Firewall rules (ssl allow to internal Exchange-IPs) are configured and work. In the monitor I can see the traffic passing the firewall. Nothing blocked.

My Problem: In the guest wifi I cant retrieve the e-mails from the mailbox. BUT only with the native iOS-App. For example the outlook-iOS app works and other android devices work as well....

Sometimes I can configure the the Mailbox and see the blue hook. And sometimes not... But retrieving mails never worked from the internal guest network. External works perfect...

Microsoft Remote Connectivity Analyzer says everything is ok. But the tests are only for the external ips....

Powershell Test-ActiveSyncConnectivity also ok and no errors.

Exchange 2019 CU11

Firewall: PaloAlto

any ideas?

Thanks a lot!!!!!

I want to confirm if this issue only occurs to ios mail app and this issue occurs randomly? If this issue occurs randomly, it may be related network issue, and based on your description, this issue does not seem to have much to do with exchange, you may need to seek experts on network or Apple ios mail for their assistance.

For more details: Current issues with Microsoft Exchange ActiveSync and third-party devices