Alternately, as I think about this...
You could also create a simple form that collects the visitors email address and message to send on to the client.
The banner would link to the form.
And you could supply both a click through and a mail conversion rate.
My understanding is that the mailto method doesn't always work.
Personally for ads like that I've fulfilled it by uploading the banner creative as a contributed image and making the clickthrough link to that image. I set the asset to delete after the run of the banner.
Then in the caption for the image I have a "Click here to email ..." statement.
I've not gotten any complaints doing it this way. Not sure if the salepeople are passing on our workaround details either.
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