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Constant Contact is unable to control the features and formatting of an email that is forwarded. This is a function of the web mail client on the receiving end. After the recipient receives the email and chooses to forward it there is nothing that Constant Contact can do to prevent them from making changes. That said, if you would like you can add a link to the webpage version of that email. That would provide a link within the email that goes to a version which cannot be changed. Click Here for instructions on adding a webpage version link.
I hope that this addresses your concerns but please let us know if you have additional questions. Thank you for using Constant Contact, have a great weekend!