When the marketing team sends an email, they can add a filter to the smartlist that excludes anyone who was sent the sales email, using the filter "Not Was Sent Sales Email."
When sales sends the email, it's a little more tricky, but there are two options:
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Change something about the email in MSI that shows it's clearly not a mass marketing email - add customization or simplify the layout. This way, even if they receive the email for a second time, it looks more like a personalized sales follow up than the same marketing eblast twice
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Have sales use the "Add to Campaign" action rather than "Send Marketo Email." When sales adds a lead to the campaign, put a filter in the smart list that excludes anyone who was sent the marketing email by using the "Not was sent email" filter.