Using Vertical Response with Litmus
Vertical Response tends to send out two separate emails for test sends, which sends one email for the HTML version and one email for the plain-text version.
Since the '@emailtests.com' address that appears on the 'New Email Test' page is temporary and is meant to receive one email, this can sometimes be problematic with these double test emails send from Vertical Response.
You may find that using the static email address (available on the Extras tab in your account) produces better results for your test emails sent from Vertical Response. Your static email address is permanent and reusable. Using the static address will create two separate tests: one for the plain-text version and one for the HTML version of your email. You'll likely see these as two different 'versions' of the same email inside your Litmus account.