An issue happened the other day where we had a particular ActiveSync device where the email would occasionally remain in the outbox. To troubleshoot ActiveSync devices in detail, you need to enable ActiveSync debug logging for the user account. See below.
Enable ActiveSync debug
logging on a mailbox:
Set-CASMailbox user1 -ActiveSyncDebugLogging:$true
Confirm that debug logging
is enabled:
Get-CASMailbox user1 -ActiveSyncDebugLogging | fl name,*logg*
Reproduce issue then output
log file to clipboard:
Get-MobileDeviceStatistics
-Mailbox user1 -GetMailboxLog:$true | clip