This issue occurs if the mailbox has multiple primary address types, and their prefixes are all uppercase letters (such as "SIP" or "NOTE").
This issue occurs because Microsoft Exchange ActiveSync (EAS) takes any type of primary address as a primary SMTP address, as long as it has the appropriate format. Therefore, an incorrect email address that is prefixed by all uppercase letters might be used in the EAS client.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
You can run the following Set-CASMailbox cmdlet to enable the Exchange ActiveSync logging:
Set-CASMailbox alias-ActiveSyncDebugLogging:$trueAdditionally, you can run the following Get-ActiveSyncDeviceStatistics cmdlet to obtain the log of the affected account:
Get-ActiveSyncDeviceStatistics -Mailbox alias -GetMailboxLog:$true -NotificationEmailAddress YourEmailAddress