As long as they know how to use it, you should be fine. The one thing I'd be careful with is running very large mailings. Since they go from Marketo, it's ok, but if they want to do more than 100-200 at a time, maybe it's better to let you run it.
Also, if they send large campaigns with their name and email on it, they risk getting dumped into Spam more often, which means even their clients may have to dig in the spam box to find their emails later.
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