Apple Mail App with multiple exchange accounts connectivity errors

Thank you panchososa694 for responding to this post/discussion. Like Microsoft's answer it did not really guide me to a resolution. Your still correct, it was a Microsoft issue.

We resolved the Apple iOS Mail App Exchange sync/connectivity issue yesterday evening.  I contacted Apple Support.  I first spoke with Kyle who listened to the details of the issue and troubleshooting steps we have already performed.  He immediately escalated to an Enterprise support engineer.  I was connected to Kathryn who asked me additional questions.  One question particularly was “How is the Exchange account setup for these users?”.  When I explained it is pushed down by our MDM she suggested we test the Apple iOS Mail App without the MDM.  This led me to start troubleshooting our On Prem MDM and identifying the root cause.  I ended up engaging my network engineer.

Root Cause:

Microsoft had recently added additional IPs to outlook.office365.com.  Our On Prem MDM Sentry server proxying the

Exchange traffic did not have all of the IP address on the Firewall whitelisted.  This would explain the intermittency.  The Firewall dynamically updates for these types of changes but not fast enough.  We ended up getting the update IP range for outlook.offfice365.com and manually updated the Firewall for the MDM Sentry Mail proxy server.

Hi panchososa694,

One of my users I had him download and install the Outlook App for iOS. He was able to email me the screenshot of the Apple Mail iOS App error from the same iPhone X. The one thing in common amongst the users having his issue is that they have multiple Exchange accounts registered with the Apple Mail iOS App. I'm not ruling out the potential of a Exchange issue. I'm reaching out to the community to see if anyone else is experiencing the same issue with the Apple Mail App.

I got a call back from Microsoft Support. "Microsoft do not support any other third party

applications"... We did some more testing. We have users experiencing the issue with just one Exchange Account registered on their Apple iOS device(s). These same users get the sync errors on WiFi & LTE. Most of the users are on the latest iOS version. I tested a third party App on a Android device w/o issue. The issue seems to be isolated to the Apple iOS Mail App connecting to our O365 Exchange Tenant. I was advised to contact Apple Support to toggle logging for the iOS Mail App. Help!