Is it common for e-mails to get rebuilt during transmit?

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http://serverfault.com – I'm having a problem where some user's of my web site complains they're receiving empty e-mail messages. After comparing the content and e-mail header of their e-mails with the ones I get I'm leaning onto the conclusion that their mail server somehow changes the content. Spam filter or virus filter maybe? I can't pinpoint that any of them exists through their mail header though. I've tested on GMail and Exchange 2010 with more-or-less the same header results. Delivered-To: [email protected] Received: by with SMTP id p4csp162755oex; Wed, 16 Jan 2013 05:01:10 -0800 (PST) X (HowTos)