You are expecting an important email from someone, he just confirmed that the mail is already sent but the message is nowhere to be found in your inbox. After sometime the mail finally arrives.
Emails are usually delivered in a second or two so why did it take so long for this particular message to reach your mailbox? Was it due to the attachments? Or was there a problem with your mail server?
You can usually find the reason for the delay in the email message itself. Open the message inside Gmail and choose Show Original to view the message headers.
Now copy the full message header to the clipboard and paste it into Google Apps’ Toolbox. This tool will analyze the email’s path and, based on the timestamps when it passed the different machines, it will tell you the exact source that is responsible for delaying the message delivery.
The toolbox is part of Google Apps but it should be able to analyze messages from other Outlook and other email programs as well because the message headers have a standard format.