Imagine you want to make an online payment, and your bank sends you a one-time code via email confirming your identity. It takes more than ten minutes to receive this code, and you want to know why it took so long. This page discusses how to determine such delay as well as allow you to analyze any message by inspecting its headers.
Emails, by design, travel through multiple SMTP servers before reaching the final destination. Let's call these intermediate SMTP servers a "hop". Delays can occur at any hop; therefore, the first step is identifying the hop that took the longest. Consider the image below as an example:
In the example above, four SMTP servers come into play, and a delay could occur at any stage.
Receivedheader, is what is important for our purpose, which contains a timestamp when the message was received at that hop.