Email headers contain a timestamp for every point at which the message is transferred to a different server.
Reviewing the timestamps in the email headers will reveal which server is responsible for a delay in email delivery.
Obtain the full headers of your email message:
How to Find Email Headers
When reading email headers from top to bottom, they tell the story of how a message traversed the internet backward. In other words, the recipient server information is at the top, and the sender server information is at the bottom.
Review each line that starts with Received: or X-Received: and note the timestamp of each Recieved header.
Compare the timestamps and calculate the difference in time between each one.
The difference in time between the timestamps equals the amount of delay that was introduced in the transfer of the message at that point.