Now that Blackbaud have released Query for Webview, you can now use a Query, instead of Lists in your Mailchimp Data Update processes.
When doing so, you might base your criteria/segmentation to output an email address as that is marked as Primary. So for example, a Constituent Record could contain multiple email addresses such as Email, Personal Email, and Business Email, but Business Email is the one that is marked as 'Primary'.
This is all fine, until you run a Raiser's Edge Update Process which requires you to map to a specific email Phone Type. This could overwrite the exiting Phone Type if they are not the same or create a duplicate record if you are try to match on email address and not Constituent ID. In the example above, Business Email could be changed to Personal email, which might confusing things in the future.
With these issues in mind, we suggest you create a new Phone Type just for using with Mailchimp such as "Email (Mailchimp)' and ensure that is always marked as your Primary email address. If the same email address appears twice on a record, but with different Phone types, you will at least understand the nature of the email address (i.e. it is a work email and one suitable for Mailchimp).