Have a rather complex question here... 

We are sending mass emails to our Leads as part of a marketing campaign. I want to make sure that we do not send emails twice to the same lead. Therefore, I created a drop-down field called "Intro Email" that we can mass update upon sending out the emails to select leads. My question - Is there a way to set up workflow automation to auto-update this field as we send the Leads a mass email?

If you have any other recommendations as far as best practices for these email campaigns, it would be much appreciated. I have checked out Zoho Campaigns, but do not understand it very well.

Thanks,

Trevor