If messages are taking longer than 5 minutes to arrive in the recipient's Inbox, the delay might be happening due to a problem ...

If messages are taking longer than 5 minutes to arrive in the recipient's Inbox, the delay might be happening due to a problem with their email client. For help fixing Outlook connection problems, read the following article: Fix Outlook problems

To see the full log of how Office 365 processed your message, you can use the Message Trace tool.

Filing a support ticket won't solve this issue if it is due to a problem that's happening outside the Office 365 service.