Microsoft 365 ← Knowledge Base
Outlook Not Connecting / Sending-Receiving Mail
Problem
Outlook desktop or mobile shows a connection error, or mail is not sending or arriving.
Symptoms
- "Disconnected" or "Trying to connect" in the status bar
- Mail stuck in the Outbox
- New mail not arriving despite webmail showing it fine
Possible Causes
- Expired or revoked authentication token
- Corrupted local Outlook profile or OST file
- Conditional access policy blocking the client
- A genuine Microsoft 365 service incident
- Mailbox storage quota exceeded
Diagnostic Steps
- Check whether webmail (outlook.office.com) works normally — this isolates the desktop client specifically
- Check the Microsoft 365 admin center or status page for an active incident
- Check the account's mailbox size against its quota
- Try Outlook in Safe Mode to rule out a misbehaving add-in
Commands & Tools
Test-NetConnection outlook.office365.com -Port 443Get-Mailbox -Identity user (admin, PowerShell)
Step-by-Step Solution
- Sign out and back in to refresh the authentication token
- If webmail works but desktop Outlook does not, repair or recreate the Outlook profile
- Rename/rebuild the OST file to rule out local data corruption
- Check and clear mailbox quota if storage is full
- Check Conditional Access policies if this is a managed device recently changed
Verification
Outlook shows "Connected", new test mail is sent and received successfully, and no errors appear in the status bar.
Common Mistakes
- Rebuilding the whole profile before checking if it's a wider M365 service incident
- Not checking mailbox quota as a simple, common cause
- Ignoring a recently installed add-in as a possible cause
Escalation Criteria
Escalate to a Microsoft 365 admin if the issue affects multiple users simultaneously, or if Conditional Access is suspected.
Prevention
Monitor mailbox quotas proactively, keep Outlook and Office updated, and review Conditional Access policy changes before wide rollout.