Hi John,
I can't think of a simple solution to this issue based on your description. Trying to enumerate all of the possible Referer hostnames that provide webmail doesn't look feasible.
Is there any pattern you could use in the URI to match the email campaign URIs? If so, you could add a check on the URI to send traffic to the pool.
Also, the redirect value should be a fully qualified URL like http://example2.com. Using "example2.com" will be interpreted as a local reference by the browser and would cause the client to request whatever URL the were redirected from with example2.com appended.
Aaron