What metadata gets stripped by mail clients?

When you reply to an email or forward it, your mail client strips out most of the behind-the-scenes routing information. It keeps what the recipient actually needs to see (subject, body, visible headers like From/To/Date) and removes everything else. What gets removed: internal routing headers (most Received lines), spam score annotations, tracking pixels, internal server names, authentication result headers, and vendor-specific debugging fields.

Why mail clients do this: privacy and security. If every forwarded email carried all its original routing data, you'd be broadcasting internal server IPs, infrastructure details, and security filter results to whoever receives it. Gmail, Outlook, and Apple Mail all strip differently, but the goal is the same. They share only what's needed to continue the conversation.

What stays intact: the visible headers (From, To, Subject, Date), the message body, attachments, and usually the most recent Received header (the one that shows where the email entered your inbox). Authentication headers like DKIM-Signature and SPF results get stripped because they're no longer valid once you modify or forward the message.

Why this matters if you're debugging delivery: when someone forwards you a problem email, you're not getting the full picture. The headers that would tell you why it landed in spam or which server rejected it are already gone. If you need to troubleshoot delivery, ask the recipient to send you the original message as an attachment (not a forward). That preserves all the routing metadata intact.

And if you're looking at headers trying to figure out what went wrong with delivery, check out our free Email Header Analyzer. It parses the full routing path and flags authentication issues. (Just make sure you're analyzing the original message, not a forwarded copy.)