Troubleshoot mail flow issues (20–25%)
Troubleshoot Exchange Online mail flow issues
• review and interpret message headers
• review and interpret message trace results and policies associated with those results
• determine whether a transport rule or conditional routing rule is affecting mail flow
• identify rules that are evaluated and policies that are applied when sending or receiving email
• troubleshoot issues where users cannot send or receive email and no NDR is generated or displayed
• troubleshoot issues where mail destined for one tenant is incorrectly routed to another tenant
• troubleshoot delivery delays
Client Access Rules in Exchange Online | Microsoft Docs
A mobile device intermittently can't connect to Exchange Online (microsoft.com)
Exchange Online Device access rules |