How do I troubleshoot connection issues in Outlook for Windows?

To troubleshoot connectivity issues in Outlook, follow the steps listed below one by one, and see if the problem persists after each step. 

WARNING: Tests should be run while the user is experiencing connectivity issues.

The Outlook status bar might be showing "Connected to Microsoft Exchange" or "Connected", but may not sync properly, or may show the "Trying to connect..." or "Disconnected" status.

• If the Outlook status is Working Offline, open the Send/Receive tab and disable the "Work Offline" option.

Check access to Webmail (OWA)

• Check if you can log in to Webmail (OWA). If you cannot log in to Webmail and get one of these messages:
• "Login or password incorrect." Verify your password is correct. If necessary, reset your password by opening a support ticket.
• "Server is unavailable." Clear the browser cache, then try to login to Webmail again.

Run Outlook in Online mode

• Run Outlook in Online mode: 
• Go to File (Tools) > Account Settings 
• Double-click on Exchange email address > uncheck Use Cached Exchange Mode > save the changes 
• Restart Outlook
• Read the Knowledge Base article on How To Enable Cached Exchange Mode In Outlook for more information.

• If there is an improvement in Outlook performance in Online Mode, it means the Outlook local cache file (.OST) is corrupted. (Note: You may need to reorganize your mailbox so that folders contain not more than 5,000 items each as recommended in the Knowledge Base article How do I improve performance in Outlook for Windows?.)
• You can recreate the Outlook profile.
• You can delete and recreate the OST file.

• Run Outlook in Safe Mode: quit Outlook, navigate to Start > in Search field type in outlook.exe /safe (mind the space between .exe and /safe) > and press Enter.

• If the performance is better, disable the unnecessary Add-ins in Outlook. Read the Knowledge Base article What do I do when the body of messages is missing in Outlook for Windows? to find instructions on removing add-ins.

• Press CTRL key and right-click the Outlook icon in the notification area
• Select "Connection Status"

• In the Exchange Server Connection Status window, you can view information about the current connections Outlook has established to the Exchange server. 
• Provide the support technician with the screenshot of SMTP Address, Display Name, Proxy Server, Status, and Avg Resp
• If the steps above did not help, provide the support technicianwith the results of the following tests:
• Description of the issue(s)
• Presence of issue(s) in Webmail
• Differences in Outlook performance in safe mode and online mode
• "Test Email Autoconfiguration" results
• Outlook and Windows versions
• Public IP address you're connecting from