Headers contain the full history of a message and settle most arguments.
HOW TO SEE THEM
In webmail, open the message and look for an option such as View Source or Show Headers. In Gmail, open the message, use the three-dot menu, and choose Show original. In Outlook, open the message and look under File > Properties for internet headers.
WHAT TO LOOK FOR
Received lines, read from the bottom upwards. Each server that handled the message added one, with a timestamp. Comparing them shows where any delay occurred. Authentication-Results, which shows whether SPF, DKIM and DMARC passed. This is the single most useful line for deliverability problems. Return-Path, which is the true envelope sender and often differs from the visible From address in forged mail. Message-ID, which uniquely identifies the message if you need to ask us to trace it.
WHY IT MATTERS
Headers tell you whether a message really came from where it claims, and whether the delay was at our end or theirs. Both are frequently assumed and rarely checked.
