I'll pile on here to agree with what everyone's said so far.
The root of the issue here is most likely what happens to an email once it hits your inbox. Generally the way the email is coded for sending from Marketo to "inbox unknown" is that it has to be able to be received by all the different inboxes and interpreted as the same thing. This often means redundant code or funny little work-around the Outlook, Gmail, etc like to see in an email but other inboxes don't care about. Once the email hits a user's inbox, there's often a bunch of the code that gets "sanitized" from the original code b/c they all act like a bunch of spoiled children who don't want to share their toys and only accept code the way they want it and get rid of the rest. For this reason, the email that you forward IS NOT the same as the email you receive when sent from Marketo.
Typically the solution here is to use the "Forward to a Friend" link which will send a net-new email from Marketo to the recipient rather than sending along the one you just got which has been "interpreted" by your Email Service Provider once it hit your inbox.
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