If you wish to disable syncing of e-mails sent outside of PersistIQ, simply follow the directions below. 

Begin by accessing the Salesforce integrations page in PersistIQ,
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Locate Salesforce and click on the Configure button.
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On the left side menu, under "Activities", select "Settings".

Un-select the options for, "You or a teammate emails a prospect outside of Persist IQ" and "A prospect replies to an email unrelated to a PersistIQ campaign."

You are all set.