Emails Are Slow to Load or Do Not Load

We recently (within the last 3 weeks) deployed a fleet of iPads (iPad 9.7's, iPad Air's, iPad Pros 11-inch 3rd gen, iPad Pros 12.9-inch 3rd gen) that are all:

• Wi-Fi only
• Running iOS 12.1.1.
• Apple DEP enrolled
• Managed by an MDM solution

Some customers are reporting poor performance when they try to open an email using the native mail client:

1. They tap an email to open it
2. The email shows the sender's name, who it was address to, the email subject and time, but the body of the email is blank showing 'Loading' with a loading wheel.
3. When in this state it either:
   1. Takes a very long time for the email body to load, OR
   2. The email body simply never loads even after being left for an hour.
4. However:
   1. If they close and reopen the mail app, it sometimes works but isn't 100% reliable
   2. If they click 'See More' it will immediately show everything, but 'See More' is not available on every email for a yet to be determined reason; Also they shouldn't have to click it in order to view their email.

The following has been tried without success:

1. It's not unique to a specific email or emails within a specific folder or path (e.g.: folder tree)
2. Re-enrolled the device in our MDM solution
3. Retired the device in our MDM solution & rebuilt the iPad
4. From the iPad itself, reset the device, retired the device in our MDM solution a& rebuilt the iPad
5. Issued a brand new iPad
6. Mail is configured by our MDM solution which by default routes mail through the MDM solution. We created a new account that bypasses that connecting directly to Exchange.
7. Upgraded from 12.1.1 to 12.1.2

So far, the Outlook mail app seems to be working where the built-in Mail app is failing but it's not clear why.

Any thoughts on what the next steps might be to troubleshoot this further?

Thanks in advance

Julius