As a Microsoft Outlook users, when you Microsoft Office together with Microsoft Office 365, you may experience one or more of the following symptoms, 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, or Outlook responds slowly when you try to open email messages. To have better assistance in order to resolve the slow performance in the Microsoft Office 365, dial Outlook customer service number or visit https://outlook.live.com/ and choose a reliable Outlook expert. 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. If you are unable to find the suitable customer help for Outlook to resolve the Outlook slow performance, you better contact Microsoft Outlook technical support.