When you use Microsoft Outlook together with Microsoft Office 365, you experience one or more of the following symptoms:
Outlook responds slowly when you try to open email messages.
When you send an email message, the message sits in the Outbox folder for a long time.
Outlook responds slowly when you try to insert an attachment. Frequently, you receive the following warning message:
Outlook is retrieving information from the server
This issue may occur for many reasons.
This issue may be caused by an Exchange Online server issue. In this case, sign in to the Office 365 portal by using your administrator credentials, and then click service health to determine whether other customers are experiencing the same issue. If other customers are experiencing the issue, the issue should end when the service-interrupting event is resolved.
Also, determine whether any local network issues might be causing the problem. This includes issues that affect your local proxy server, firewall, or Internet service provider.
To troubleshoot this issue, follow these steps:
Make sure that the latest updates for Outlook are installed. For more information, see the following Microsoft Knowledge Base article:
2625547 How to install the latest applicable updates for Microsoft Outlook (US English only)
Test to learn whether the issue is resolved. If the issue persists, go to step 2.
Try the methods from the following Microsoft Knowledge Base articles, as appropriate for the version of Outlook that you're running:
For Outlook 2013:
2652320 Outlook 2013 or Outlook 2010 not responding, hangs, freezes or stopped working
For Outlook 2010:
2695805 How to troubleshoot performance issues in Outlook 2010
For Outlook 2007:
940226 How to troubleshoot performance issues in Outlook 2007
If the issue persists, go to step 3.
Try the following:
Use the Microsoft Office Configuration Analyzer Tool (OffCAT) to determine whether too many mail items are located in the email account’s critical data folders. For more information, see the following Microsoft Knowledge Base article:
2768656 With many items in a Cached mode .ost file or .pst file folders, Outlook performance issues may occur
Disable Skype for Business Online (formerly Lync Online) integration. If Outlook is integrated with Skype for Business Online in your Office 365 environment, the integration may affect Outlook’s performance. To disable Skype for Business Online integration with Outlook, follow these steps:
Open Lync, click the gear icon () in the upper-right corner, point to Tools, and then click Options.
Click Personal, and then clear the check boxes in the Personal information manager area.